Hyper Stabilized Bullets - Jim Boatright

Lowlight

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#1
Jim Boatright has forwarded another article for your consideration.

I will post his articles in the resource section moving forward

I actually have a sample of the new bullet Jim speaks about in the article. David Tubb was using it, and as noted, it's from Warner Tool.

IMG_0243.jpg
 

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Feb 10, 2017
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#2
Does this mean we can expect a Frank Galli test of a .308 with a 1-6 twist barrel and some monolithic solids? Because, that would be pretty cool to see the real world benefit to it
 

Fursniper

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#4
Well, I read the entire attachment & while I managed to glean a few bits of knowledge and understanding from it, especially about twist rates, I also developed a major migrain while doing so lol. Perhaps if I had a degree in phyiscs it would have been less painful but wonder if someone could dumb it down a bit for us average riflemen that are trying to wrap our heads around some of these "darker mysteries" involved in the challenges of overcoming & ultimately achieving more success with ELR!
 

Lowlight

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#6
I guess that pretty much blows a hole in the recommendation by Bartlien to stick with a gain of around 3/4 of an inch!
Doesn't do anything of the kind, stick to claiming ignorance

The 13 -5.4 was for a specific bullet that was very, very long and this barrel is so old it has no other purpose as it won't work for anything

Pretty dumb trollish statement especially when you look at the results
 

THEIS

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#7
Hi,

In very simplified terms :)

The "sooner" we can get monolithics stabilized the less downrange affects of yaw, pitch, etc and since monolithics can be twisted faster than jacketed bullets....spin them fast so they can achieve hyper stabilization as close to the muzzle as possible.

Kind of reminiscent of Lutz Moeller saying 7 twist for 375s back decade ago.


Sincerely,
Theis

Edited so nobody confused Lutz with Litz
 
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THEIS

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#9
The shorter bullet is the one they are talking about, the longer one was meant for the aggressive gain twist, huge difference in 338s here.
Hi,

LL that long bullet looks like an old Zathilius (sp) Associates bullet....is that what it is or is it one of the super heavies from GS?

Sincerely,
Theis
 

Jeffvn

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#12
Yes that is a ZA .338 pill on the left. Frank I think you and I are the only ones who have a 5.4" exit twist .338 barrel. Mine has maybe 250 rounds through it. Sits in my garage up on the wall.
 

Fursniper

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#13
Doesn't do anything of the kind, stick to claiming ignorance

The 13 -5.4 was for a specific bullet that was very, very long and this barrel is so old it has no other purpose as it won't work for anything

Pretty dumb trollish statement especially when you look at the results
So Bartlien has recommended a 3/4" gain, you claim you've had (I have to assume) good results from a much faster twist, therefore I assume perhaps Bartlien's recommedation is perhaps too conservative based on results you've had with your fast GT barrel, make a comment on that & get called out as a dumb troll? WTF? Seems a bit harsh Frank. How would I know your barrel was made for a very unusually "one off" long bullet & therefore was a very special case scenario? I thought you had commented on this to point out that faster twists based on your substantial experience might be more beneficial than previously thought & was "taking that to heart". I certainly never meant to imply that Bartlien didn't know what that they recommending, only that I thought your experience had proved otherwise, ie, that considerably faster twists could possibly yield positive results, especially after reading the latter part of the attachment you posted that spoke to faster twist rates than I had seen before for 338 projectiles. I don't know what "results" you were referring to, I assumed (perhaps incorrectly, if so my bad) they were "good" results otherwise I was at a loss as to why you would mention it.
 
Feb 11, 2017
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#14
I would like to point out that the bullet pictured is not a Flatline in any way. This a bullet designed by James Boatright and has not yet been introduced for sale by anyone. The only similarity it has to the Flatline is that the same guy (me) makes them. And since the cat is already out of the bag, David was asked to test them. Clearly depicted by the photograph, he has added his patent pending ogive modification to some. Just thought I should clarify this for you.
 

Lowlight

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#15
You're assuming too much and attempting to put words in my mouth.

1. You never see me talk about this barrel, except in very specific terms, like with this very long bullet. I always caveat my discussions around it.

2. I did not contradict anyone when the discussion of 3/4 Gain Comes up - Many would call that a Clue - you immediately went for it.

3. The fact you read nothing about my, "results" should be another clue. I did not go into details because it was an unrealized project. It never fully blossomed. You jumped the shark.

4. Yes, this was a very specific project, however, because of the data in the article, it does provide an opportunity for this barrel, which like Jeff is doing nothing but collecting dust. The fact it's a gain twist and ends in 5.4 means it might work in this context.

This only works with Solid bullets, Jacket's won't survive, another clue.
 

Jim Boatright

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#17
Most rifle bullets eventually achieve hyper-stable flight if they fly far enough. Gyroscopic stability Sg increases throughout the flight. By achieving it right out of the muzzle, however, the rifleman is minimizing yaw-drag on his bullets during the critical first several coning cycles of flight where the bullet's coning "angle-of-attack" is usually largest and where the "potential drag force" (q*S) is always its very highest.
The new ULD bullet development is branching into two different designs: 1) a base-drilled version for shooters transitioning into shooting monolithic bullets using rifles built for lead-cored bullets, and 2) an "all-out" ELR bullet which will only be used in special extremely fast (20 calibers per turn) twist-rate barrels. The base-drilling greatly improves initial gyroscopic stability from conventional twist-rate barrels, and allows elastic expansion of the rear driving band at peak chamber pressure to seal the combustion gasses perfectly even in less than perfect barrels. The "all out" Mark II version is solid copper and has its secant-ogive nose lengthened from 3.0 calibers to 3.2 calibers. The 252-grain 338-caliber Mark II prototype bullet is 1.850 inches long and might have a BC(G1) approaching 1.000 at Mach 2.5 in hyper-stable flight.
Gary Schneider is building me a 28-inch HV-profile 338 barrel with P5 rifling at 7.0 -inches (21.2 calibers) per turn for my "short-range" BR test rifle in 338 LM. I am hoping to see it in a few weeks. He will have to make a new 338-caliber button for the 6.6-inch twist-rate.
 
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THEIS

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#18
Hi,

It is very nice to see older "concepts" merge with modern technology/understandings to bring us improvements :)

Cheytac & Associates had the "concept" of lighter weight for caliber projectile (305gr Dominator) to achieve higher MV but did not pair it with a fast enough twist rate to achieve hyper stabilization.

Lutz Moeller recommended a 6.5 twist rate for his LM105 .338 projectiles but did not have the MV needed to achieve hyper stabilization because the projectiles were heavy for caliber.

Gerard at GS saying that his projectiles would handle a stability factor of 3+.

Klaus had the "concept" of lighter weight for caliber paired with crazy high MV (Mach IV 375/BMG wildcat) but did not pair it with a fast enough twist rate to achieve hyper stabilization.


Jim...do you know what calibers you will be releasing the MkII version in?

Edited To Add:

The 252-grain 338-caliber Mark II prototype bullet is 1.850 inches long and might have a BC(G1) approaching 1.000 at Mach 2.5 in hyper-stable flight.
Jim, I understand you are having a barrel spun up for these to provide field data but what is PRODAS telling you in regards to the bleeding off of the BC? As you mentioned...that is great BC number at Mach 2.5! Is PRODAS showing a steady bleed off curve or is there a "minimum" flight velocity you are recommending?
This is where the 305gr Dominator .408 projectile sucks at....it sheds BC and Velocity drastically.

Sincerely,
Theis
 
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TripleBull

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#19
Have the extremely fast twist rate barrels been tested enough to know what kind of life to expect from them? Do they require different cleaning/maintenance routines compared to conventional twist barrels?
 

Lowlight

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#20
I have not seen a need to clean more or that they affect the barrel life. Plus barrels are expendable, barrel life is more a product of cartridge, not the barrel. My 7 twist 260 is shorter, so I lose a bit of velocity, and I dropped the bullet weight a bit so I get back that velocity and it still works. I did not do it shoot heavies, I shoot the 136, and I am working up a 123gr load to see. That puts my velocity back, while adding in the spin for stability at extended ranges.

I know this is all fairly new and everyone is trying to add in that Spin to bring up the stability factor, but the bullet is the weak link here. In order to go this aggressive, you have to use Solids. In order for this to work correctly with jacketed bullets, you have to go Gain Twist like we are talking about in the other thread. The Gain Twist movement we are reviving is the true answer to all this, but with a mix of jacketed and solids.

For jacketed bullets, it's pretty safe to go 3/4 Gain but the service rifle guys are going hyper-aggressive, but when you read about it, their velocity is down a bit because their barrels are shorter. They are doing that 13 to 6 twist rate, which gives them a wider variety of bullets to shoot from a single rifle. They start light and go heavy at distance. The entire spread works for them.

The balance to strike here is velocity vs twist so you don't deform or destroy the bullet before it hits the target. The shortcut to this balance is a gain twist barrel.
 
Feb 10, 2017
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#21
So, can you effectively get a higher twist rate at the muzzle with gain twist from jacketed bullets? Like, for a typical 6.5, most people are shooting a 1-8, or a 1-7. Like Lowlight said, the service rifle guys are going 13 to 6. Is there something stopping you from doing this? Is the problem with jacketed bullets from the jolt of imparting a lot of spin really quickly, or is it the RPM's that cause problems too?
 

Lowlight

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#22
It's jacket tech, you have to look at what bullets you want to shoot. Bullets with thinner jackets will deform quicker.

In typical range situations, where you are shooting supersonic there is really no need to hyper stabilize this stuff. So the gains into the 7s with a 6.5 is plenty already, I see no reason to go more. For me, an 8 to 7 twist 6.5 is about perfect. Bartlein is currently recommending 3/4 so 8 to 7.25 is good too. Or 8.25 to 7.5 to mix in the light bullets.

But for ELR Shooting where 1/2 the distance can be at subsonic speeds, then going hyper stable makes sense. When we shot the Ritter Stark Event, we used a Factory Rifle and Factory ammo at 4000 yards. Results were terrible, but the bullet went transonic at 1800 yards. At 2600 yards we had better results both Ray (TBAC) and I went 4 out of 5 at this distance. If they used a gain twist, results might have been better, a 9 to 8 or something close to that would probably work well. The only other option was completely switching ammo.

RPMs can be the problem, there used to be pictures of bullets creating pinwheels of lead on paper with them coming apart. Dave Tooley posted them during a barrel test he did. I think there is a line, not super fine, but still, a line that can go over.
 
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damoncali

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#23
Have the extremely fast twist rate barrels been tested enough to know what kind of life to expect from them? Do they require different cleaning/maintenance routines compared to conventional twist barrels?
It's not the barrels, it's the bullets you need to worry about. If you've ever seen someone shoot 90 grain .224 bullets, one of the challenges is to keep them stable without blowing up the bullets. Solids mitigate this problem because it's the jackets that fail. The other issue with really fast twists is that it degrades accuracy by increasing sensitivity to aerodynamic jump. At long range, and especially ELR, this is effect is lost in the noise, and trumped by improved ballistics of minimizing yaw drag by getting the bullets to point ever so slightly more into the air flow. We typically don't see people paying attention to yaw drag because 99% of ballistics calculators roll it into the BC, and it's really not that relevant to short range shooters. Litz's book where he tested BC vs Sg demonstrates the same effect at a high level (BC and Sg are correlated- to a point), although Jim's write-up takes it a step further by showing *why* you see this effect and puts a finer point on it.
 

damoncali

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#24
Well, I read the entire attachment & while I managed to glean a few bits of knowledge and understanding from it, especially about twist rates, I also developed a major migrain while doing so lol. Perhaps if I had a degree in phyiscs it would have been less painful but wonder if someone could dumb it down a bit for us average riflemen that are trying to wrap our heads around some of these "darker mysteries" involved in the challenges of overcoming & ultimately achieving more success with ELR!

The dumb version is that BC is composed of two contributors to drag. The first - CD0 - is the drag the bullet experiences along its axis - when yaw is zero. The second is CDa, which is "yaw" drag. It's the drag attributable to the bullet's yaw relative to the oncoming air flow. If a bullet is perfectly pointed into the airflow at all times, total drag is just CD0, which is what we want - it's as low as it can get. In real life, there's aways a bit of yaw. This is a bit of an oversimplification, but it hits the high points.

The amount of yaw at any given point in a bullet's trajectory is dependent on the spin rate of the bullet. There's a lot of pitching and swerving going on, but Jim shows basically that at very fast twists, you can minimize that (and CDa), and therefore get very close to ideal drag - pure CD0.

When we measure BC, we measure the combination of the two drags. And he's cautioning bullet designers against hyper-focusing on CD0, because that requires long skinny noses and boattails, that require ever increasing twist rates to stabilize (and minimize CDa). Ultimately it is out of the bullet makers hands what rifle they get shot out of, so some clarity and care in this area is wise for this type of "ULD" bullet, which has a very specific application (ELR).

What we're seeing is the trends that you would expect as we tend to shoot longer and longer ranges. At very short range, you don't care about yaw drag. What you want is pure accuracy, which means slow twits and stubby bullets. As you go further out, ballistics starts to matter more, and so drag (and CDa) start to matter more. At some point, the accuracy you gained with the slow twist is lost to poor ballistics. At ELR ranges, ballistics utterly dominate, so that's why really fast twists with really long bullets rule the day, and why it doesn't matter if your ELR rifle can shoot 0.25 minute groups. The things you do to get short range benchrest levels of accuracy are counterproductive for ELR.
 

TripleBull

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#25
It's not the barrels, it's the bullets you need to worry about. If you've ever seen someone shoot 90 grain .224 bullets, one of the challenges is to keep them stable without blowing up the bullets. Solids mitigate this problem because it's the jackets that fail.
I'm assuming we're talking solids based on reading in this thread and elsewhere. But that still leaves me curious about barrel life.
 
Feb 20, 2013
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#27
What we're seeing is the trends that you would expect as we tend to shoot longer and longer ranges. At very short range, you don't care about yaw drag. What you want is pure accuracy, which means slow twits and stubby bullets. As you go further out, ballistics starts to matter more, and so drag (and CDa) start to matter more. At some point, the accuracy you gained with the slow twist is lost to poor ballistics. At ELR ranges, ballistics utterly dominate, so that's why really fast twists with really long bullets rule the day, and why it doesn't matter if your ELR rifle can shoot 0.25 minute groups. The things you do to get short range benchrest levels of accuracy are counterproductive for ELR.
(y)

Just would add to that that Total Drag is more than just the first two drag terms, and Jim's proposal of drilling a small hole at the base looks promising. Not "base bleeding" but akin to. Reducing Base Drag will ultimately tend to achieve better dynamic stability thus better ELR ballistics.
 
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damoncali

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#28
(y)

Just would add to that that Total Drag is more than just the first two drag terms, and Jim's proposal of drilling a small hole at the base looks promising. Not "base bleeding" but akin to. Reducing Base Drag will ultimately tend to achieve better dynamic stability thus better ELR ballistics.
I don't know of the reasoning behind the drilled base - I assumed the intention there was to play around with the weight distribution rather than something to do with base drag?

But yes, I'm differentiating only between yaw drag and axial drag, both of which have multiple sources. Cd = CD0 + yaw x CDa , mathematically speaking.
 

damoncali

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#29
Jim, I was rereading your paper, and I'm curious about this bit:

"I suspect that in developing his similarity-based McDRAG program for projectiles usingogive head-shapes, McCoy was inadvertently predicting the total coefficients of drag CD’sfor projectiles flying with significant, but unmeasured, coning angles because of themeasurements data-set to which he was fitting his McDRAG calculations using multivariatelinear regressions."

What makes you think that?
 

bohem

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#30
Hi,

It is very nice to see older "concepts" merge with modern technology/understandings to bring us improvements :)

Cheytac & Associates had the "concept" of lighter weight for caliber projectile (305gr Dominator) to achieve higher MV but did not pair it with a fast enough twist rate to achieve hyper stabilization.

Lutz Moeller recommended a 6.5 twist rate for his LM105 .338 projectiles but did not have the MV needed to achieve hyper stabilization because the projectiles were heavy for caliber.

Gerard at GS saying that his projectiles would handle a stability factor of 3+.

Klaus had the "concept" of lighter weight for caliber paired with crazy high MV (Mach IV 375/BMG wildcat) but did not pair it with a fast enough twist rate to achieve hyper stabilization.


Jim...do you know what calibers you will be releasing the MkII version in?

Edited To Add:



Jim, I understand you are having a barrel spun up for these to provide field data but what is PRODAS telling you in regards to the bleeding off of the BC? As you mentioned...that is great BC number at Mach 2.5! Is PRODAS showing a steady bleed off curve or is there a "minimum" flight velocity you are recommending?
This is where the 305gr Dominator .408 projectile sucks at....it sheds BC and Velocity drastically.

Sincerely,
Theis
The 375 Warner actually works unlike the Klaus BMG wildcat. The 361s are moderate in speed compared to what a 305gr 40 caliber bullet would do but at 3600fps and an average, measured BC that approaches 1.0 it holds the initial launch velocity quite well.

Hollow base bullets are not new, even in copper solids. There was someone, already mentioned in this thread, that was doing that with some of his solids about 8 years ago at least, maybe longer. There are all sorts of effects that come from drilling assholes in bullets. I tried it with some flatlines about 6-7 years ago. I did it as a fix to get some 7 caliber long 7mm bullets to fly properly with a 4 caliber nose length. The failures did help find a problem in the original stability calculations and we did get them to work eventually with some assholes in them... fun pill that was. 190 class 7mm made from copper and the BC ran north of 0.9 but it was unsuitable for anything short of a 7 twist even with a hollow tail.

I also had the issue of unburnt rifle powder getting stuck in the base which helped explain the erratic MV's and flight at times, we tried filling it with pistol powder and getting a bump in MV, that was fun too. There are some very touchy things that happen to a hollow tail bullet once yaw angle is introduced. The hollow cavity can set up a resonance and "ring" with a shockwave that is over the critical velocity for the speed of sound in the cavity at reduced pressure.

I admire Mr. Boatright's approach, it's not an easy road to travel.

If someone has a 338 with a 6.5 or 7 twist and the ability to accurately measure a BC I'd like to see what a 256 Flatline BC changes from the 10 twist BC we measured acoustically and Hornady did with doppler to what it would do when the twist rate is bumped 30% or more. As it stands the avg G1 with a 10tw was measured to appx 0.89, correcting that to M2.5 the BC at high speed would be appx 0.95ish, closer to 1.0 at M2.8-M3.0
 

Jim Boatright

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#31
Hi,

It is very nice to see older "concepts" merge with modern technology/understandings to bring us improvements :)


Jim...do you know what calibers you will be releasing the MkII version in?

Edited To Add:



Jim, I understand you are having a barrel spun up for these to provide field data but what is PRODAS telling you in regards to the bleeding off of the BC? As you mentioned...that is great BC number at Mach 2.5! Is PRODAS showing a steady bleed off curve or is there a "minimum" flight velocity you are recommending?
This is where the 305gr Dominator .408 projectile sucks at....it sheds BC and Velocity drastically.

Sincerely,
Theis
1) Thanks, Theis. I was not really aware of a lot of this history. I have never shot competitively beyond 200 yards. I do have several years experience in 100-yard benchrest shooting, but mostly at club level. I am too old and do not see well enough to shoot much anymore. My reasons for investigating ELR shooting are an attempt to guide today's riflemen into more productive thinking lines of thinking which will advance to sport more efficiently. I decided to design a new ELR bullet because I saw design errors in past efforts in that direction. I patented that design mainly so that nobody else could do so and claim credit for my work.

2) The new ULD bullet design is easily scaled to any desired caliber. Dan Warner is making them for David Tubb to test. Chuck Pierce is also testing them here in my 105-yard indoor benchrest range. I have sent Dan the details necessary to program his CNC machines to make copper prototype base-drilled Mark I ULD bullets in 338 caliber (225-gr) and in 6.5 mm (105-gr). Without base-drilling the 338 bullet weighs 242 grains. Dan is sending some of those to David for testing. David asked for a heavier 338 bullet, so I sent Dan the details for the 252-gr Mark II version to make for David. I have also completed scaling the Mark II version to 375-caliber using my bullet design spreadsheet tool. I settled on making it for 0.366-inch bore and 0.375-inch groove ID's. It will weigh 344 grains and might have a BC(G1) of 1.001. The Mark II bullets simply use a 3.2-caliber secant ogive in place of the 3.0-caliber ogive of the Mark I design.

3) I only wish I did have access to PRODAS. I have gotten Don Carlucci at Picatinny Arsenal to make some runs for me in the past, but he is not answering my messages lately. The PRODAS Preprocessor, which calculates all the aeroballistic coefficients necessary for subsequent 6-DoF runs from projectile design specs, is highly proprietary. It probably started with McDRAG when John Whyte, Sr., was at BRL, and continued onward. They went private and Arrow Tech rents it to the Army and other military researchers.

Jim Boatright
 
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Jim Boatright

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#32
Jim, I was rereading your paper, and I'm curious about this bit:

"I suspect that in developing his similarity-based McDRAG program for projectiles using ogive head-shapes, McCoy was inadvertently predicting the total coefficients of drag CD’s for projectiles flying with significant, but unmeasured, coning angles because of the measurements data-set to which he was fitting his McDRAG calculations using multivariate linear regressions."

What makes you think that?
While programming the supersonic parts of McDRAG into my bullet design spreadsheet, I came to understand Bob McCoy's thinking a little. I used to do similar studies in FORTRAN years ago. The problem with any linear regression is that the results are only as good as the sample data-set available. Not having yet discovered coning theory, I do not believe McCoy fully realized the limitations of his Army test data. Just an educated guess on my part.

Jim Boatright
 

Jim Boatright

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#33
Hollow base bullets are not new, even in copper solids. There was someone, already mentioned in this thread, that was doing that with some of his solids about 8 years ago at least, maybe longer. There are all sorts of effects that come from drilling assholes in bullets. I tried it with some flatlines about 6-7 years ago. I did it as a fix to get some 7 caliber long 7mm bullets to fly properly with a 4 caliber nose length. The failures did help find a problem in the original stability calculations and we did get them to work eventually with some assholes in them... fun pill that was. 190 class 7mm made from copper and the BC ran north of 0.9 but it was unsuitable for anything short of a 7 twist even with a hollow tail.

I also had the issue of unburnt rifle powder getting stuck in the base which helped explain the erratic MV's and flight at times, we tried filling it with pistol powder and getting a bump in MV, that was fun too. There are some very touchy things that happen to a hollow tail bullet once yaw angle is introduced. The hollow cavity can set up a resonance and "ring" with a shockwave that is over the critical velocity for the speed of sound in the cavity at reduced pressure.

I admire Mr. Boatright's approach, it's not an easy road to travel.



I did not write this paper to discuss bullet designs, but I will be happy to do so. The original design shown in the patent drawings showed a fairly large-diameter, shallow base-drilling which could be optionally deepened for inserting and seating a lead core. Tests showed that powder packing was not a problem, but sometimes the thin walls of the boat-tails fractured and tore off. I calculated that about 12,000 psi of unanticipated differential internal pressure (inside vs outside the boat-tail) was need to cause this fracturing right in front of the chamber. I don't think acoustic over-pressure can explain this failure. I suspect some type of "hydraulic ram" effect in the pressurized unburned powder mass. So, I reduced the drill size and solved the failure problem. Eventually, I settled on a drill of 0.5-calibers in diameter, with a drill shoulder depth about half-way under the rear driving band. Fired bullets recovered from Chuck Pierce's swimming pool clearly showed perfect obturation of the driving band between the rifling marks just as far forward as the drill shoulder depth. The internal pressure ducting allows that region to expand elastically by up to 1 thousandth of an inch into hard contact with the insides of the grooves. I was testing in a 10-inch twist 6-groove Bartlein 338 barrel. The smaller, deeper base-drilling removes 6.7 percent of the most troublesome bullet mass, improving initial Sg at 3000 fps from my 10-twist barrel from 1.34 to 1.50. I believe the improved obturation available is worthwhile, at least in some barrels.

We are now seeing short-range accuracy problems with these 225-gr 338 bullets, versus the un-drilled 242-gr bullets; 1-inch 5-shot groups instead of 0.15-inch groups. I suspect powder-packing once again. I plan to enlarge the rear portion of the holes for a tight interference fit with the next larger size of common plastic (Delrin) ball bearings ($0.02 each) for our next firing tests of these 225-gr bullets. My hope is that the Delrin ball will be forced deep into the hole at peak chamber pressure, acting as a piston and ducting that base pressure internally for the elastic expansion mentioned. Then, when the bullet exits, I hope the remaining compressed air ahead of it will cleanly eject the now under-sized plastic slug. Sort of an internal discarding sabot, which should clear any packed powder out also. I f this idea does not work, we will probably abandon the base-drilling idea and its advantages for short-range shooting.

I expect the base-drilled bullet to fly about like the solids in exterior ballistics, but only testing will tell us that.

Jim Boatright
 

Jim Boatright

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#34
The dumb version is that BC is composed of two contributors to drag. The first - CD0 - is the drag the bullet experiences along its axis - when yaw is zero. The second is CDa, which is "yaw" drag. It's the drag attributable to the bullet's yaw relative to the oncoming air flow. If a bullet is perfectly pointed into the airflow at all times, total drag is just CD0, which is what we want - it's as low as it can get. In real life, there's aways a bit of yaw. This is a bit of an oversimplification, but it hits the high points.

The amount of yaw at any given point in a bullet's trajectory is dependent on the spin rate of the bullet. There's a lot of pitching and swerving going on, but Jim shows basically that at very fast twists, you can minimize that (and CDa), and therefore get very close to ideal drag - pure CD0.

When we measure BC, we measure the combination of the two drags. And he's cautioning bullet designers against hyper-focusing on CD0, because that requires long skinny noses and boattails, that require ever increasing twist rates to stabilize (and minimize CDa). Ultimately it is out of the bullet makers hands what rifle they get shot out of, so some clarity and care in this area is wise for this type of "ULD" bullet, which has a very specific application (ELR).

What we're seeing is the trends that you would expect as we tend to shoot longer and longer ranges. At very short range, you don't care about yaw drag. What you want is pure accuracy, which means slow twits and stubby bullets. As you go further out, ballistics starts to matter more, and so drag (and CDa) start to matter more. At some point, the accuracy you gained with the slow twist is lost to poor ballistics. At ELR ranges, ballistics utterly dominate, so that's why really fast twists with really long bullets rule the day, and why it doesn't matter if your ELR rifle can shoot 0.25 minute groups. The things you do to get short range benchrest levels of accuracy are counterproductive for ELR.
Jim Boatright:

I agree with almost all of what you say here, D. But it is not the yaw-drag coefficient CDa itself which we need to minimize; only it effect when multiplied by q*S*a^2 and added into q*S*CD0. We seek to do this by minimizing the coning angle-of-attack (a) in hyper-stable flight. I have no quarrel with bullet designers unintentionally increasing CDa while reducing CD0. I have done so myself.

Also, being able to fire 0.25-inch 5-shot groups at 100 yards is what a mathematician would call a "necessary but not sufficient condition" for long-range accuracy. I hope to demonstrate 0.100-inch 5-shot grouping from my 338LM in my indoor 105-yard range. That would correspond to 1-inch at 1000 yards.

Many things which are very important at long range (like muzzle speed uniformity) are much less important at 100 yards. Cross-wind sensitivity is almost as important to the short range BR shooter because his aggregate group sizes are scored in ten thousandths of an inch. I hope eventually to make a 65-grain 6 mm ULD bullet available for the 6mm PPC crowd.
 

Jim Boatright

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#35
A few general comments;

I do not intend to get into discussions of gain-twist barrels, as I do not think you have to go there, at least not immediately. However, I do recommend that you think carefully about selecting the pattern of the rifling. I like the advantages of cut-rifling in short-range, slow-twist barrels. We shoot soft, jacketed match bullets which "upset" easily to seal the bore. Copper-alloy bullets certainly will not do this. I really think David Tubb is onto something using Gary Schneider's "Pull-button P5" rifling with copper bullets, especially at very fast rates of twist. The P5 rifling pattern takes care of the obturation problem. European "polygonal" rifling in hammer-forged barrels is also promising for copper-alloy bullet use. I don't know how long those can be made, though.

I hope riflemen will start using rifle barrels with "20 calibers per turn" twist rates with all monolithic copper-alloy bullets at any shooting distances, not just mine, if I ever release them for sale. I will not do so until they have been thoroughly tested. If long-nosed copper-alloy ULD bullet are not hyper-stabilized very early out of the muzzle, they will shoot poorly. Fired with an initial gyroscopic stability (Sg) of about 3.0, they will show their true potential.

The main advantage my bullets will eventually offer is their "self-aligning" feature which is independent of the chamber and throat design selected and of the rifling pattern and twist-rate selected. I do not think that most riflemen fully appreciate just how serious the problem of in-bore yawing actually is. In-bore yaw causes both lateral throw-off (right out of the muzzle) due to CG offset and aerodynamic jump due to large initial ballistic yaw (again right out of the muzzle). These randomly oriented deflections affect the entire trajectory of each fired bullet. Short-range BR shooters have pretty much solved this in-bore yaw problem by "jam seating" their short, tangent-ogive bullets and by maintaining absolute minimum "safe" case-neck and ball-seat clearances around the afterbody of the bullet. Those solutions do not translate well to military sniping or "fixed ammo" target matches, for example. Many shooters are constrained to using SAAMI or CIP-standard chambers. My bullets center themselves mechanically at two separate locations before engraving the rifling: the front of the "bore riding" shank and the rear of the 7.5-degree conical ramp at groove ID. Those two locations are 1.3-calibers apart longitudinally. Each bullet is thus centered and aligned with the axis of the bore. That is why I expect much greater accuracy from them in all types of rifle shooting.

Jim Boatright
 

Jim Boatright

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#37
Just let me know if you want to try some of my 242-gr ULD's as well. You could push them pretty hard for another great test of their mechanical strength. I think the rear driving band will be up to the challenge. Dan Warner is going to make more of them soon. Then, before much longer he might have the 252-gr Mark II version ready. Neither will be base-drilled, and both should be quite accurate.
 

damoncali

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#38
While programming the supersonic parts of McDRAG into my bullet design spreadsheet, I came to understand Bob McCoy's thinking a little. I used to do similar studies in FORTRAN years ago. The problem with any linear regression is that the results are only as good as the sample data-set available. Not having yet discovered coning theory, I do not believe McCoy fully realized the limitations of his Army test data. Just an educated guess on my part.

Jim Boatright
It just so happens that I read and implemented a version of McDrag last month, and I had some of the same concerns, although to be honest, I never looked into it in detail. I did note that at least some of his sources appear at first glance to be on top of the problem, but I was left wondering how good their measurements of CD0 were back in 1948, and if there might be a slight under-prediction of BC. Although the validations given by McCoy don't even claim to be accurate enough to matter - that was my takeaway, and why I never bothered to look into it.

I'm not familiar with your "coning motion" theory. I assume that if PRODAS validates it, then it is mathematically compatible with a standard 6DOF model?
 

bohem

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#39
1) Thanks, Theis. I was not really aware of a lot of this history. I have never shot competitively beyond 200 yards. I do have several years experience in 100-yard benchrest shooting, but mostly at club level. I am too old and do not see well enough to shoot much anymore. My reasons for investigating ELR shooting are an attempt to guide today's riflemen into more productive thinking lines of thinking which will advance to sport more efficiently. I decided to design a new ELR bullet because I saw design errors in past efforts in that direction. I patented that design mainly so that nobody else could do so and claim credit for my work.

2) The new ULD bullet design is easily scaled to any desired caliber. Dan Warner is making them for David Tubb to test. Chuck Pierce is also testing them here in my 105-yard indoor benchrest range. I have sent Dan the details necessary to program his CNC machines to make copper prototype base-drilled Mark I ULD bullets in 338 caliber (225-gr) and in 6.5 mm (105-gr). Without base-drilling the 338 bullet weighs 242 grains. Dan is sending some of those to David for testing. David asked for a heavier 338 bullet, so I sent Dan the details for the 252-gr Mark II version to make for David. I have also completed scaling the Mark II version to 375-caliber using my bullet design spreadsheet tool. I settled on making it for 0.366-inch bore and 0.375-inch groove ID's. It will weigh 344 grains and might have a BC(G1) of 1.001. The Mark II bullets simply use a 3.2-caliber secant ogive in place of the 3.0-caliber ogive of the Mark I design.

3) I only wish I did have access to PRODAS. I have gotten Don Carlucci at Picatinny Arsenal to make some runs for me in the past, but he is not answering my messages lately. The PRODAS Preprocessor, which calculates all the aeroballistic coefficients necessary for subsequent 6-DoF runs from projectile design specs, is highly proprietary. It probably started with McDRAG when John Whyte, Sr., was at BRL, and continued onward. They went private and Arrow Tech rents it to the Army and other military researchers.

Jim Boatright
Are you basing your calculations of BC on the McDrag formulae or have you expanded it out to CFD solutions?
 

Jim Boatright

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#40
It just so happens that I read and implemented a version of McDrag last month, and I had some of the same concerns, although to be honest, I never looked into it in detail. I did note that at least some of his sources appear at first glance to be on top of the problem, but I was left wondering how good their measurements of CD0 were back in 1948, and if there might be a slight under-prediction of BC. Although the validations given by McCoy don't even claim to be accurate enough to matter - that was my takeaway, and why I never bothered to look into it.

I'm not familiar with your "coning motion" theory. I assume that if PRODAS validates it, then it is mathematically compatible with a standard 6DOF model?
Actually, I first pinned down the exact nature of the bullet's coning motion from some 200 yards of simulated flight of the 168-gr 30-caliber SMK, together with an image of the matching wind-axes plot of spin-axis motions, supplied by Bryan Litz about 10 years ago from his own 6-DoF simulator which I imagine he did as an aero engineering project in college. It should closely match McCoy's 6-DoF simulator. I digitally low-pass filtered out the precession rate to recover the mean trajectory (on 0.1msec centers, as I recall) in drift and drop, and then subtracted those values point-by-point to find the "coning motion of the CG." This showed the coning motion of the CG to be 180 degrees out of phase with the slow-mode motion of the spin-axis.

From there, I did the usual orbital mechanics work to show that the coning was an isotropic harmonic oscillation. McCoy had part of this, but not quite all. I showed that the motion is torsional about the coning apex ahead of the bullet and is powered both by aerodynamic lift and drag forces. The formulated coning distance (behind the apex) and coning rate (same as slow-mode precession rate) all work out. I could send you an updated coning theory paper. One version used to be here on Sniper's Hide back when it was hosted by somebody else. Gustavo Ruiz (Patagonia Ballistics) posted it for me, or at least sent it to Frank. I used coning theory to allow analytic calculation of crosswind aerodynamic jump, yaw of repose angles, and spin-drift several months ago. I am currently trying to simplify dynamic stability a little using coning theory and classical mechanics. Not there yet, though.

The PRODAS runs I got later from Don Carlucci at Picatinny had data reported only on 1 msec centers, so it is a little harder to see the 60 hz slow-mode motions, but the effects predicted by coning theory are there. The coning theory calculations of CWAJ and spin-drift match those integrated by PRODAS quite well. Those 1000-yard PRODAS runs are for the M118 Special Ball round at 2800 fps from some rifle with an 11.25-inch twist barrel as I recall. They ran no wind, 10 mph crosswind, and LH and RH spin direction combinations without Coriolis for about a dozen total runs. We were looking for any asymmetries with crosswind direction versus twist direction as reported by David Tubb and others, but every thing was perfectly symmetrical according to PRODAS.

Jim Boatright
 

Jim Boatright

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#41
Are you basing your calculations of BC on the McDrag formulae or have you expanded it out to CFD solutions?
I use McCoy's McDRAG supersonic drag estimates as a design aid because my rifle bullet design uses the same projectile design parameters McCoy fitted in McDRAG (including a circular arc head-shape), and also because I have no access to any CFD capability. I ran my implementation of McDRAG for the M549 155 mm artillery projectile and for the G7 reference projectile as sanity checks. I would not claim any drag data which had not been measured in firing tests. Eventually, I would like to see some test-firing data from a Doppler radar system at Yuma or Dahlgren. I expected McDRAG calculations to be optimistic, perhaps by 10 or 20 percent. When Davids Oehler System 88 reported 12.9 percent lower drag than McDRAG had predicted, I had to figure how that could be. The only reasonable answer seemed to be hyper-stability (a name I just made up). Until the bullets start printing "nose high" on paper targets, I refuse to term them "over-stabilized."

Jim Boatright
 

jbailey

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#42
Has the ULD bullet described in US Patent '155 and pictured by Lowlight above actually been fired thru a rifle bore yet?

And I am correct in assuming that the key novel design features of this new ULD "self-aligning" bullet are the:
(1) conical ramp 109
(2) drive band 111 (which is 1.02-1.034 reference diameters)
(3) hollowed out rear cavity 117
?
1519406995886.png

It seems to me 101, 103, 105, 107, 113 and 115 are all obvious from prior art, or am I missing something on these?
 
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Jim Boatright

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#43
Has the ULD bullet described in US Patent '155 and pictured by Lowlight above actually been fired thru a rifle bore yet?

And I am correct in assuming that the key novel design features of this new ULD "self-aligning" bullet are the:
(1) conical ramp 109
(2) drive band 111 (which is 1.02-1.034 reference diameters)
(3) hollowed out rear cavity 117
?
View attachment 6877267

It seems to me 101, 103, 105, 107, 113 and 115 are all obvious from prior art, or am I missing something on these?

I think that is a fair summary, J. I had 20 of them turned several years ago from free-machining" C360 brass. The ones fired were terrible. The boat-tail had fractured and torn off. They would have withstood centrifugal force had they not been fractured by some unexpected internal (non-hydrostatic) pressure, possibly a "ram" effect with the compressed powder mass. After simply plugging the holes with Devcon-F, they held together. These drawings are from 2015, before any real development testing had begun. The reduced diameter base-drilling is mechanically stout enough, but now may suffer from powder packing. The Sg improvements and internal pressure ducting are too useful to give up easily, though.

Jim Boatright
 

THEIS

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Nov 27, 2017
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#44
I really think David Tubb is onto something using Gary Schneider's "Pull-button P5" rifling with copper bullets, especially at very fast rates of twist. The P5 rifling pattern takes care of the obturation problem.
Hi Jim,

I just spoke with Gary this morning in great lengths in regards to exactly this subject. Particularly the P5s he did back in the mid 90s for the USN Tac-50s, as well as the results David is getting. He just finished a couple more 375s for David.

I am going to have Gary spin me up a 375 with P5 rifling profile in 7 twist but in the hardened alloy he uses for the 50s so that I can revive (with modifications) the Klaus' 375/BMG concept now that propellants have advanced. Reamer will FINALLY be here next week...ordered in November.

Sincerely,
Theis
 

FlaShooterG

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Jan 21, 2018
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#45
Has the ULD bullet described in US Patent '155 and pictured by Lowlight above actually been fired thru a rifle bore yet?

And I am correct in assuming that the key novel design features of this new ULD "self-aligning" bullet are the:
(1) conical ramp 109
(2) drive band 111 (which is 1.02-1.034 reference diameters)
(3) hollowed out rear cavity 117
?
View attachment 6877267

It seems to me 101, 103, 105, 107, 113 and 115 are all obvious from prior art, or am I missing something on these?
Now for the real stupid question of the day. I realize this is probably like bringing a knife to a gun fight, but golf uses dimples to stabilize the flight of the ball. Any value in doing that in the area between 113 and 115 for stabilization?
 

bohem

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#48
I use McCoy's McDRAG supersonic drag estimates as a design aid because my rifle bullet design uses the same projectile design parameters McCoy fitted in McDRAG (including a circular arc head-shape), and also because I have no access to any CFD capability. I ran my implementation of McDRAG for the M549 155 mm artillery projectile and for the G7 reference projectile as sanity checks. I would not claim any drag data which had not been measured in firing tests. Eventually, I would like to see some test-firing data from a Doppler radar system at Yuma or Dahlgren. I expected McDRAG calculations to be optimistic, perhaps by 10 or 20 percent. When Davids Oehler System 88 reported 12.9 percent lower drag than McDRAG had predicted, I had to figure how that could be. The only reasonable answer seemed to be hyper-stability (a name I just made up). Until the bullets start printing "nose high" on paper targets, I refuse to term them "over-stabilized."

Jim Boatright
Yes, McDrag is optimistic in most cases, especially when it comes to bullets which have more than 1 bearing surface type feature such as a driving band. It is not only the coning approach but I suspect several additional minute factors that are nearly impossible to solve for without a well bounded CFD solution. There are also near sonic effects that McDrag cannot handle.

When you said the M2.5 BC is approaching 1.0, is that the BC specifically at M2.5 or a BC measured with a bullet that was launched around M2.5 and then measured at some distance downrange like the O88 handles, such as 1000yd or 1500yd?

I like this part:

I refuse to term them "over-stabilized."[/qupte]

I whole-heartedly agree.
 

Jim Boatright

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#49
Yes, McDrag is optimistic in most cases, especially when it comes to bullets which have more than 1 bearing surface type feature such as a driving band. It is not only the coning approach but I suspect several additional minute factors that are nearly impossible to solve for without a well bounded CFD solution. There are also near sonic effects that McDrag cannot handle.

When you said the M2.5 BC is approaching 1.0, is that the BC specifically at M2.5 or a BC measured with a bullet that was launched around M2.5 and then measured at some distance downrange like the O88 handles, such as 1000yd or 1500yd?

I like this part:

Hello, Bohem;
I tried to keep my bullet design clean and simple to minimize secondary shocking--always keeping nose-drag, base-drag, and boundary layer (BL) skin friction in mind a lot. I defined "1.0 calibers" to be the bore ID plus 0.0002-inch to help reduce nose-drag versus using the groove ID +0.0002 (machine tolerance). I wanted the laminar BL flow over the ogive to trip reliably into turbulent flow at the 4.5-degree break angle at the base of the 3.2-caliber secant ogive (with RT/R = 0.5). Bullet OAL is 5.60-calibers. I wanted the turbulent BL to slide cleanly off the sharp-cornered BT toward a compression point 3.2 calibers behind the base of the BT. I selected the 7.5-degree cone angle for the ramp up to groove ID with both BL flow and bullet alignment guidance in the chamber throat in mind. It all seems to work. The rifle bullet design ends up looking somewhat similar to the M549 155 mm artillery projectile.

The Mach 2.5 airspeed comes from David's test of the 225-gr version of the 338-caliber bullets. His range is 995.7 yards and time of flight in a 21 mph tailwind was 1.068 seconds at 65 degrees F. Correcting for air distance travelled in 1.068 seconds yields a time-average airspeed of Mach 2.46. His Oehler System 88 with square-array acoustic mics at target distance (i believe) calculated an average BC(G1) of 0.794 for 5 shots and 2325 fps (Mach 2.070) arrival speed. His average MV was 3378 fps (Mach 3.008). So, Mach 2.46 sounds about right for this 1000-yard BC measurement. Oehler still uses the old ASM atmosphere. I used ICAO with McDRAG. Air density is 0.5 percent greater with the sea-level ICAO atmosphere. McDRAG had estimated a BC(G1) of 0.703 at Mach 2.5. If we attribute the remaining 12.4 percent difference in BC's to David's bullets being hyper-stabilized at an initial Sg of 2.75 from his 7.5-inch twist Schneider barrel, we could reasonably scale up other McDrag estimates similarly for other hyper-stabilized rifle bullets until they can be test-fired. That's why I said Mach 2.5 BC(G1) approaching 1.00.

By the way, to estimate initial Sg for those base-drilled bullets, I scaled the effective length of the base-drilled bullets from their actual length by the square root of the Iy/Ix ratios after/before drilling, and then used that effective length in McGYRO and Don Miller's Sg estimators. That is, I used the radius of gyration ratios ky/kx to scale the effective length reduction. I calculate the mass properties by numerical integration in my spreadsheet bullet design tool. Dan's copper bullet-making material seems to have a density of 8.84 grams per cc.

Jim Boatright

Jim Boatright
 
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Jim Boatright

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#50
Hello again;

I don't know if anyone caught it, but I slipped a mental cog when I commented in the hyper-stability paper that the rifling helix angle for a "20-calibers per turn" twist rate is 360/20 = 18 degrees. It should have said 180/20 = 9 degrees. Bit by confusing radius and diameter again. I don't often think in terms of rifling helix angles as barrel makers would.

Jim Boatright
 
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