Spin Drift

Jim Boatright

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Feb 21, 2018
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Drift and jump have nothing to do with each other. Drift is due to a yaw of repose, which is defined as a quasi-steady state yaw (basically, the average yaw) to the side caused by gravity and spin. That yaw is relative to the airflow, which already accounts for wind. YOR imparts a lift force, which causes drift. Wind has nothing to do with it. The angular value grows with distance because the lift force is always present.

Wind deflection is caused by drag, not lift. Drop is obviously caused by gravity, and slowed by drag. This is why wind and drop are related, and both are covered by BC.
[Wind deflection is indeed caused by aerodynamic drag. The drag force on the flying bullet is always "downwind" relative to the apparent wind experienced by the moving bullet. The apparent wind (Wa) is always W - V, a three-dimensional vector difference. The magnitude of this drag force is about 1.1 pounds for a 168-gr 30-caliber rifle bullet at 2800 fps. This is 46 times the weight of that rifle bullet. Crosswind sensitivity is determined by this force ratio. At a given airspeed and for the same bullet design aways flying nose forward in the same atmosphere, the drag force varies with the frontal cross-sectional area of the bullet; ie, with the square of its caliber. The weight (m*g) of the bullet varies with the cube of its caliber, all else being equal. So, this Drag/Wt ratio varies inversely with caliber, all else being equal. That is why large-caliber bullets have less wind sensitivity than smaller-caliber bullets.]

Aerodynamic Jump is due to an angular misalignment and/or rotational velocity upon exiting the bore. It is a transient effect that happens immediately after launch, and it causes an angular change in the line of departure. The only thing it has to do with wind is that wind can be the source of the initial tipping.
[There are many types of aerodynamic jump phenomena based on several different causes. One type is due to the just-fired bullet entering the ambient atmosphere having a non-zero aeroballistic yaw or yaw-rate. This could be caused by a bad muzzle crown or a destabilization within an improperly designed muzzle brake, for examples. Another type (CWAJ) could be caused by a perfectly launched rifle bullet encountering a crosswind instead of still ambient air. Yet another type of AJ occurs any time the flying bullet encounters a step-change in the direction of the apparent wind it is seeing. In every case, the aerodynamic jump deflection is 90 degrees advanced in the sense of the rifling twist direction (CW for RH twist) from the cross-track direction of the flight disturbance.]

CG Jump is similar to AJ, but it results from a lateral offset CG of the bullet (due to manufacturing tolerances). Think about the direction a CG is moving when it leaves the bore. If it's offset from the centerline, it will leave at an angle determined by its helical path down the bore. Wind plays no part here, but your bullets do. This is one reason FMJs suck (AJ is another).
[To avoid confusion, I prefer to call this type of trajectory deflection "lateral throw-off." It occurs whenever the CG of the bullet followed a helical (spiral) path as the bullet traversed the bore of the rifle barrel. The bullet "flies off on a tangent" as soon as it is no longer constrained by the barrel. Lateral throw-off can be cause by firing bullets which were manufacture out of static balance or by firing perfectly made bullets with "in-bore yaw," or by both effects together, as occurs with jacketed, lead-cored VLD match bullets.]

Neither Aerodynamic Jump nor CG Jump grow angularly with distance. There is no down range force that causes those effects.

They can all be calculated if you have good bullet aerodynamic data. The calculations are not hard, but gathering the data is.
[These statements are correct, Damon.]
Comments added in-line in [brackets].
Jim Boatright
 
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V-Ref

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I do not think I am looking at it wrong --

Like you apparently, I have been shooting ELR for long time.

If one says they can "see" and out wind call SD, it would have to be the first shot. The second shot is a correction and ALWAYS has everything in it. Form there, we are ALL just looking at the variables and similarities in wind from the previous shots. I am not saying that SD/AJ/CE/EV are not real, just usually hidden in other variables.

In ELR, hit percentages go way up when the correction is clear and ultra fast, and short. Up .2 / Left .3, whatever, or the actual new setting might be the only things said without math just using the ruler..

Hell, I am not even saying your not as good as the best wind callers in the world, but I have yet to see one that really can out call SD in switchy wind. It is a very tall statement to say that one is so good they can see the actual value and then out call SD. Sure lots of us have said for years, the the model seems way too aggressive, but the problem is even over thousands of ELR rounds, we still can't pin point the "exact" amount. OK, Jim B's 2% seems to make sense, more so than any Ballistician's offerings, but even then, it is very hard to pull out of the real wind.. That's why we need smart guys like him :).

"…all models are approximations. Essentially, all models are wrong, but some are useful." — George E.P. Box, Norman R. Draper

Reminds me when some people were running around here and saying they "had to account for CE" because they were such great shots. Obviously most is hidden in velocity variations and wind; It was more of an ego exorcise than anything else.

Sure put the values in, but no need to focus so much energy on it.
I appreciate your posts here. Don't take this as argumentative. I had to grab a post to quote from, yours was handy.

The other side of this argument is who is really good enough to say that AJ/SD/CE ISN'T THERE. Who can actually call a no bs zero wind condition at ELR distances. A very very few that have access to fully instrumented mil ranges. If a shooter has the opportunity to have ever popped smoke at say 3 distances between them and the ELR target durin thier "No Wind" condition, they'd be more confused on the wind hold then without the smoke...even during thier "No Wind" condition/session.

If that's counter to your experiences, that's ok, I respect that. The above are mine. The effects are real. They are there. All of it matters for the first shot as you alluded. How well it's modeled, per projectile, per system...I think that's certainly a valid point to toss it all out. Let's just all call the first shot a sighter, and roll from there.

Hopefully it's on the target :)
 
Feb 7, 2013
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I appreciate your posts here. Don't take this as argumentative. I had to grab a post to quote from, yours was handy.

The other side of this argument is who is really good enough to say that AJ/SD/CE ISN'T THERE. Who can actually call a no bs zero wind condition at ELR distances. A very very few that have access to fully instrumented mil ranges. If a shooter has the opportunity to have ever popped smoke at say 3 distances between them and the ELR target durin thier "No Wind" condition, they'd be more confused on the wind hold then without the smoke...even during thier "No Wind" condition/session.

If that's counter to your experiences, that's ok, I respect that. The above are mine. The effects are real. They are there. All of it matters for the first shot as you alluded. How well it's modeled, per projectile, per system...I think that's certainly a valid point to toss it all out. Let's just all call the first shot a sighter, and roll from there.

Hopefully it's on the target :)
No issue here. We are saying the exact same thing. Actually, most of us are.

Point being old models apered to be overestimating at shorter ranges we were fairly certain. We are sure it is real, just uncertain of the exact amount. I’ve been out many times set up waiting for first light in the desert seems like no wind, but I am sure it’s there. Solver asking for 1 full mil left or more for SD. With those kinds of old numbers it is super easy to see something is wrong.

My personal point, like yours, is none of us is good enough to call wind/no wind at those ranges to determine “exactly” what the SD is.

Thanks to continued efforts like Jim B is sharing and THEIS’s is attempting to capture, we will have a better data.
 
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Jun 26, 2012
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No issue here. We are saying the exact same thing. Actually, most of us are.

Point being old models apered to be overestimating at shorter ranges we were fairly certain. We are sure it is real, just uncertain of the exact amount. I’ve been out many times set up waiting for first light in the desert seems like no wind, but I am sure it’s there. Solver asking for 1 full mil left or more for SD. With those kinds of old numbers it is super easy to see something is wrong.

My personal point, like yours, is none of us is good enough to call wind to determine “exactly” what it is.

Thanks to continued efforts like Jim B is sharing and THEIS’s is attempting to capture, we will have a better data.
i had an absolute zero wind day yesterday. it was hot, stank and humid. shot out to 1k dialed no wind and held no wind. got hits almost every single time. no SD accounted for at all. No CE accounted for at all. but thats not really ELR and may or may not apply here, but i will say that my Ballistic calc was telling me to hold almost .5 mil for SD. i ignored that.

EDIT: correction, it was .2 not .5. I was going off memery so I double checked.
 
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Skookum

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i had an absolute zero wind day yesterday. it was hot, stank and humid. shot out to 1k dialed no wind and held no wind. got hits almost every single time. no SD accounted for at all. No CE accounted for at all. but thats not really ELR and may or may not apply here, but i will say that my Ballistic calc was telling me to hold almost .5 mil for SD. i ignored that.
First of all, let me say that I totally believe you. It still doesn't mean you can totally ignore it.

You don't say how big the target was, but if you were only getting say 0.2 mils of spin drift rather than 0.5 mils, that still the equivalent of 1 mph wind @ 1k with a 6.5 Creed. Now add to that the .5-1 mph wind that you didn't see, and now you could be missing off the edge 50% of the time depending on how your load groups at that range.
 
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Jun 26, 2012
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First of all, let me say that I totally believe you. It still doesn't mean you can totally ignore it.

You don't say how big the target was, but if you were only getting say 0.2 mils of spin drift rather than 0.5 mils, that still the equivalent of 1 mph wind @ 1k with a 6.5 Creed. Now add to that the .5-1 mph wind that you didn't see, and now you could be missing off the edge 50% of the time depending on how your load groups at that range.
Inside 800 I was shooting the little tiny IPSC silouhettes and some hangers. 900and 1k were the standard steel silhouettes we r used to. Again not denying it doesn’t exist, I just don’t think you get it until transonic ranges when stability starts becoming a factor. If I was shooting my 6.5 beyond say 1.2 Mach I would definitely be looking to add it. Yes the bigger targets are very forgiving. It could be there and yes hard to see a 6.5 hit at 1k so who knows.
 
Jun 26, 2012
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First of all, let me say that I totally believe you. It still doesn't mean you can totally ignore it.

You don't say how big the target was, but if you were only getting say 0.2 mils of spin drift rather than 0.5 mils, that still the equivalent of 1 mph wind @ 1k with a 6.5 Creed. Now add to that the .5-1 mph wind that you didn't see, and now you could be missing off the edge 50% of the time depending on how your load groups at that range.
See my edit also in my post. Makes what you say make more sense.
 

Skookum

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See my edit also in my post. Makes what you say make more sense.
In Everday Sniper, episode #12, Frank mentions that he often zeros his rifles one click (.1 mil) left. I used to zero two clicks left (.5 moa) when I was still using moa scopes. I think this approach makes good sense for anyone that just doesn't want bother with any of this. It absolutely will not hurt you at 700 yards and closer and will only help as you reach out further.
 
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In Everday Sniper, episode #12, Frank mentions that he often zeros his rifles one click (.1 mil) left. I used to zero two clicks left (.5 moa) when I was still using moa scopes. I think this approach makes good sense for anyone just doesn't want bother with any of this. It absolutely will not hurt you at 700 yards and closer and will only help as you reach out further.
wasnt he referring to elevation zero though? on a few of my rifles i set the zero stop .5 mils lower than actual zero so if im using a flatter shooter than i have zeroed i can dial down.
 

Skookum

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wasnt he referring to elevation zero though? on a few of my rifles i set the zero stop .5 mils lower than actual zero so if im using a flatter shooter than i have zeroed i can dial down.
Not the way I heard it. I did listen to it a couple of times. It is toward the end, like the last five minutes or so.

That was commonly taught back in the day. I haven't heard it mentioned much in the last ten years or so. It still makes a lot of sense for shooters that actually need to hit shit with the first shot.

Not so much for those that fire a shot, look at it with disbelief for a minute, then sit up and start finger fucking an app and re-synching thier Kestrels.
 
Jun 26, 2012
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Not so much for those that fire a shot, look at it with disbelief for a minute, then sit up and start finger fucking an app and re-synching thier Kestrels.
so true. for the most part, i try to be tech free on the range, except the kestrel for atmospherics. i have built some excel data cards that i use. after a day at th erange i then go home in the comforts of air conditioning and then i use the calculator to compare to real life and mess with it then. so much time wasted messing with stuff. shoot. hit. record.

my overall range and post range methodologies are weak. im trying to refine what i do and how much extra garbage i bring with me.
 

Skookum

Flattus Domini
May 6, 2017
830
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so true. for the most part, i try to be tech free on the range, except the kestrel for atmospherics. i have built some excel data cards that i use. after a day at th erange i then go home in the comforts of air conditioning and then i use the calculator to compare to real life and mess with it then. so much time wasted messing with stuff. shoot. hit. record.

my overall range and post range methodologies are weak. im trying to refine what i do and how much extra garbage i bring with me.
I use my G-Shock watch for station pressure. I've beat the shit out of it for 8 years now. I figure if it goes tits up then that means I probably did too. I use an IR laser thermometer to get a reading on barrel temp and ammo temp, but I have a small fob with a thermometer on it as a back up. I of course have my phone and cheap Kestrel (no ballistics just wind and temp) but I have an old M2 artillery compass for double checking azimuth and angle.

When purely gathering data, I might not hit the target a single time. Every shot gets plotted on grid paper and notes are kept for every shot. To gather good data, the instant gratification of a hit has to be supressed.
 

Lowlight

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So I shot my LH Gain Twist 8.5 to 7.75 260 and the other Tikka with 1-7 RH Twist with the same ammo on the same targets

Using my Wind Target that has .2 Mil Tynes, the wind was identical at .5 Mil holds. Same hold for both rifle with the same ammo.

Using an AB Kestrel where the drop and wind were trued via BC and using the DSF at 1125 yards, the data was perfect. However, with the 1st wind zone set to 0 and the second set for 8 MPH, the .13 of SD was clearly not right. (Sure tiny at this distance, but the windage was right at .6) So I can match up the Wind as long as I remove the SD.

Both rifles have a similar zero using my 136gr Scenar L load and the 130gr Prime Load.

130gr Prime was 2828, the software changed it to 2805fps and the BC used was.290
136gr Scenar L was 2850fps out of the 20" barrel, the 22" was a bit faster.

There is a .2 Mil offset between the Prime and the handload at 100 yards using the 20" Tikka, and no offset using the 22" LH Gain Twist, they hit in the same spot and you can interchange the data pretty easily.

The 20" Tikka is, however, a much flatter shooter than the 22" LH Gaint Twist barrel, by close to .5 mil, even less, once I flush everyting out here at home. The data was handwritten I have to translate to software now

The 136 gr load used .5 Mil of wind at 1125 where the 130gr Load used 1 Mil of wind under the same conditions.

So drift testing here seems to show barely any difference out to 1125 yards with either rifle. They trend wind wise identical, within .1 mils if not better.
 

Jim Boatright

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So I shot my LH Gain Twist 8.5 to 7.75 260 and the other Tikka with 1-7 RH Twist with the same ammo on the same targets

Using my Wind Target that has .2 Mil Tynes, the wind was identical at .5 Mil holds. Same hold for both rifle with the same ammo.

Using an AB Kestrel where the drop and wind were trued via BC and using the DSF at 1125 yards, the data was perfect. However, with the 1st wind zone set to 0 and the second set for 8 MPH, the .13 of SD was clearly not right. (Sure tiny at this distance, but the windage was right at .6) So I can match up the Wind as long as I remove the SD.

Both rifles have a similar zero using my 136gr Scenar L load and the 130gr Prime Load.

130gr Prime was 2828, the software changed it to 2805fps and the BC used was.290
136gr Scenar L was 2850fps out of the 20" barrel, the 22" was a bit faster.

There is a .2 Mil offset between the Prime and the handload at 100 yards using the 20" Tikka, and no offset using the 22" LH Gain Twist, they hit in the same spot and you can interchange the data pretty easily.

The 20" Tikka is, however, a much flatter shooter than the 22" LH Gaint Twist barrel, by close to .5 mil, even less, once I flush everyting out here at home. The data was handwritten I have to translate to software now

The 136 gr load used .5 Mil of wind at 1125 where the 130gr Load used 1 Mil of wind under the same conditions.

So drift testing here seems to show barely any difference out to 1125 yards with either rifle. They trend wind wise identical, within .1 mils if not better.
This is all quite interesting, and I am learning a lot from these learned shooter's comments. I do have one question though. I know little about gain-twist barrels and have never used one. When a barrel maker says 3/4 gain twist, I wonder if they mean that as the ratio of the initial and final twist rates, not the difference in inches between the initial and final twists? That would make the final (muzzle) twist rate 6.375 inches per turn instead of 7.75 inches per turn for LL's 8.5-inch initial twist 6.5 barrel. I called Bartlein at 5:01 PM CDT, but they had set their phones to the answering machine. Higher initial spin-rates produce a lot less spin-drift, all else being equal.
 

Lowlight

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The exit is 7.75, we did not cut the barrel down beyond threading and crowning, we cut from the chamber side

The starting spin is 8.5" and it ends at 7.75" to the Left as noted,

I use the exit value of 7.75 for the software

1.0 would be 8.5 to 7.5, though they recommend 3/4 meaning start at 8.5 and end at 7.75

My other 260 LH GT barrel is 8.25 to 7.5 in twist and the 20" 260 is a straight 1-7" twist
 

Lowlight

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If you look at those accuracy facts in the book posted above, they put more weight on bullet balance, core shift, and especially bullet roundout as potential issues of greater error, I would bet the GT barrel helps with that as the BC has increased.
 
Jun 26, 2012
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The exit is 7.75, we did not cut the barrel down beyond threading and crowning, we cut from the chamber side

The starting spin is 8.5" and it ends at 7.75" to the Left as noted,

I use the exit value of 7.75 for the software

1.0 would be 8.5 to 7.5, though they recommend 3/4 meaning start at 8.5 and end at 7.75

My other 260 LH GT barrel is 8.25 to 7.5 in twist and the 20" 260 is a straight 1-7" twist
wouldnt 8.25 - 7.75 be 1 1/4 not 3/4? or am i having a dyslexic moment?

EDIT: YEP I WAS BEING DISLEXIC. 5 out 3 of us have it you know.
 
Feb 20, 2013
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Higher initial spin-rates produce a lot less spin-drift, all else being equal.
You mean this as a general rule? Asking because in the JBM calculator as long as I increase Sg, Spin-drift goes up (keeping all the same except for twist rate), in other words a faster rate shows an ever increasing SD
 

Jim Boatright

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You mean this as a general rule? Asking because in the JBM calculator as long as I increase Sg, Spin-drift goes up (keeping all the same except for twist rate), in other words a faster rate shows an ever increasing SD
Yes, LastShot300, all else being equal, the spin-drift at 1000 yards is linearly proportional to the barrel twist in inches (or calibers). I did not expect to see that. I happen to have an elaborate spreadsheet for calculating SD at 1000 yards as published in the SD paper for eight different rifle bullets. For any bullet, I can change only the barrel twist number and the computer recalculates everything. For example, I have some 6-DoF data for the old 30-caliber 168-gr Sierra International bullet fired at 2800 fps. When I simply change the barrel spin-rate from 12"/turn to 6"/turn, the calculated SD drops from 7.1" at 1000 yards to 3.55". Same for any other bullet. Not that the old Sierra International could withstand 5200 rev/sec, but it's just a "what if" game. I was surprised to see that exact linear proportionality fall out of some rather involved formulations, even for modern copper ULD bullets.

The key phrase here is "all else being equal." I did not put the McGYRO calculations of initial Sg into that little spreadsheet, but merely hand entered the Sg values from McGYRO, so, the Sg did not change with twist-rate. However, this error points out the fact that bullet spin-rate and its Sg are NOT the same. The mass distribution of the bullet plays a huge part in determining its Sg along with its aerodynamic overturning moment which depends on air density, etc. For a given bullet, launch speed, and atmosphere, the initial Sg goes with the square of its initial spin-rate.

When I doubled the spin-rate of the bullet, I should have thought to multiply the initial Sg value by 2 squared, or 4. That change in Sg, by itself would have multiplied the SD calculated at 1000 yards by a factor of 4.345. Doubling the spin-rate as well, would cut this back down by half. So, the real effect of making both changes together would be a net increase in SD by a factor of 2.1725. You are right, I was wrong. Mea culpa.

While I am apologizing, I should also say that I should not have questioned LowLight's final twist-rate for his LH gain-twist barrel. It just seemed to me that a progressive twist increase from 8.5 in/turn to only 7.75 in/turn would be too much trouble to bother with. I would just get a straight 5.5-inch twist barrel.
 
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Yes, LastShot300, all else being equal, the spin-drift at 1000 yards is linearly proportional to the barrel twist in inches (or calibers). I did not expect to see that.

When I doubled the spin-rate of the bullet, I should have thought to multiply the initial Sg value by 2 squared, or 4. That change in Sg, by itself would have multiplied the SD calculated at 1000 yards by a factor of 4.345. Doubling the spin-rate as well, would cut this back down by half. So, the real effect of making both changes together would be a net increase in SD by a factor of 2.1725. You are right, I was wrong. Mea culpa.

.
Jim, thanks a lot for your in-depth answer.(y)
 
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