BC of bullets as they slow down- Flat vs Boattail

spife7980

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Feb 10, 2017
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#1
I was/am looking at subsonic ammo stability comparing 7 vs 8 twist on a sierra 220 round nose for the new blackout Im building. I notice that on sierras site that a 220 round nose gains BC as it slows down according to their banded numbers which is the opposite of everything I had noticed until now. I then went and looked at a 150 gr flat base round nose and it gains BC as well. So I then compared to boattail versions of equivalent weights and the boat tail versions lose BC like I was accustomed to seeing. I thought it might be due to the roundnose vs pointed but it appears that its just a flat base vs boat tail phenomenon.

220 Round Nose Flat Base- Gains
.310 @ 2600 fps and above
.335 between 2600 and 2200 fps
.378 between 2200 and 1600 fps
.410 @ 1600 fps and below

220 Matchking Boat Tail- Loses
.629 @ 2100 fps and above
.624 between 2100 and 1700 fps
.608 @ 1700 fps and below


150 Round Nose Flat Base- Gains
.200 @ 2700 fps and above
.227 between 2700 and 1700 fps
.270 @ 1700 fps and below

150 Soft Point Flat Base- Gains
.336 @ 2600 fps and above
.346 between 2600 and 1800 fps
.360 @ 1800 fps and below

150 Soft Point Boat Tail- Loses
.380 @ 2600 fps and above
.368 between 2600 and 1800 fps
.360 @ 1800 fps and below


So can anyone explain to me why a flat base gains BC as it slows down and a boat tail loses it?
 
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spife7980

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#3
Hmm, its isnt in all calibers but in some. Im trying to compare where theres a straight equal weight comparison between both styles.

Woah!
The 28 cal bullets both gained as they slow down! And the 180 gr 30 Cals all gained at least partially too!

22 Cal
55 Soft Point Flat Base- Gains & Loses

.237 @ 2800 fps and above
.239 between 2800 and 1800 fps
.230 @ 1800 fps and below

55 Soft Point Boat Tail- Loses
.250 @ 3000 fps and above
.245 between 3000 and 2000 fps
.235 @ 2000 fps and below


25 Cal
100 Soft Point Flat Base- Loses

.330 @ 2400 fps and above
.322 between 2400 and 1700 fps
.296 @ 1700 fps and below

100 Soft Point Boat Tail- Loses
.355 @ 2800 fps and above
.333 between 2800 and 1600 fps
.310 @ 1600 fps and below

117 Soft Point Flat Base- Loses
.388 @ 2500 fps and above
.383 between 2500 and 1800 fps
.362 @ 1800 fps and below

117 Soft Point Boat Tail- Loses
.410 @ 2500 fps and above
.403 between 2500 and 1800 fps
.370 @ 1800 fps and below


27 Cal
130 Soft Point Flat Base- Gains

.370 @ 2700 fps and above
.379 between 2700 and 2200 fps
.383 @ 2200 fps and below

130 Soft Point Boat Tail- Loses

.436 @ 2800 fps and above
.418 between 2800 and 2200 fps
.402 between 2200 and 1800 fps
.387 @ 1800 fps and below


28 Cal
140 Soft Point Flat Base- Gains
.377 @ 2400 fps and above
.386 between 2400 and 1800 fps
.400 @ 1800 fps and below

140 Soft Point- Boat Tail- Gains
.416 @ 2400 fps and above
.418 between 2400 and 1800 fps
.423 @ 1800 fps and below



30 Cal
180 Round Nose Flat Base-
Gains
.240 @ 2800 fps and above
.280 between 2800 and 2200 fps
.330 between 2200 and 1500 fps
.355 @ 1500 fps and below

180 Soft Point Flat Base- Gains
.407 @ 2600 fps and above
.415 between 2600 and 1600 fps
.414 @ 1600 fps and below

180 Soft Point Boat Tail- Gains & Loses
.501 @ 2700 fps and above
.506 between 2700 and 1700 fps
.505 @ 1700 fps and below

180 Match king Boat Tail- Gains
.475 @ 2800 fps and above
.496 between 2800 and 2200 fps
.494 between 2200 and 1600 fps
.494 @ 1600 fps and below




So it doesnt look like its directly tied to a Flat Base vs Boat Tail after all but it does seem to correlate to that more than anything else I can find.
 
Last edited:
Jun 26, 2012
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#4
The only explanation I offer for this is that the flat back bullets are closer in form factor to the reference G1 projectile where as the BT is not. Are there any G7 BC’s available for these?
 

spife7980

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#7
Interesting, so they think that its to make it better line up with the G1 model and not necessarily that its flying "better" as it slows. Thats just what you have to enter to line it up with the model in the calculator.
 
Jun 26, 2012
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#8
Interesting, so they think that its to make it better line up with the G1 model and not necessarily that its flying "better" as it slows. Thats just what you have to enter to line it up with the model in the calculator.
i mean the round nose blunt tale bullets were never designed to go supersonic, so the fact that the BC is higher in the subsonic range makes sense.....sorta. and its not like BT bullets are unstable in subsonic, the problem is the transition. what it does to the bullet is different every time and impossible to predict. you could fire the same round 100 times and 52 times it will tumble, 40 times it wont and the other 8 it does something else.
 

spife7980

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#10
Where is the data from?

And you mentioned a couple of twist rates if they increased the spin for a bullet going slower it would help with the BC a bit.
Its copy pasted straight off of the individual bullet profiles on sierras website https://www.sierrabullets.com/products/bullets/rifle.cfm

I started looking into whether I really wanted a 7 twist like my bolt has or if an 8 twist would be good enough since many of the AR barrels are 8 twist (8 is good enough even at 500 fps according to jbm and bergers calcs).
I dont care so much about the actual BC in this instance for the ar since they are all shit at subsonic speed, it just so happens to be what lead me to noticing the phenomena when entering numbers into the stability calculators.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#11
220 otm bt at 1015 -970fps in 16 and 10.5 barrels respectively, 300 blk.
Both stable and give good acuracy with the 1/7 twist.
I belive several ammo mfgs suggest 1/7 for the 220 subs.
Have not shot much hv in the 300 blk guns, lighter gr stuff,
Not what I bought them for.
I like to hear the crunch when using the can, fun very low recoil enjoyment.

Has been working for me for what it's worth
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#13
So if this is what I am understanding,
While shooting subs, on short yardages.

I may get a better bc from round nose flat base bullets.
I may also get better groups?

I had never considered this possibility.
Thanks!
 
Dec 4, 2002
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#14
The groups (at short/mid ranges) depend on the quality of the bullets, not the basic design.

In the past, the flat base bullets were considered "better" for short range accuracy because of the relatively ease of production. Nowadays, not so much since you can get match grade boat tails.
 

Snuby642

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Feb 11, 2017
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#15
I have read about this in 22lr match bullets.. I discounted it, probable mistake.

I'm not dealing with a transition, 1050 fps max, 30 cal, 125 yds or less.
Non expanding ammo is acceptable for this purpose.
Headshot accuracy a must in this case.
Need reliable 1 moa or better.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#16
Just to clarify, not poaching.
Rough terrain warrants the ice pic shooting for recovery of game.
Have been using 223 but is noisy in the valley.
 

spife7980

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#17
Probably the drag model does not fits the bullet very well... common with many designs.

Check this:

http://www.exteriorballistics.com/ebexplained/index.cfm

http://www.exteriorballistics.com/ebexplained/5th/24.cfm

Play with this:

http://www.geoffrey-kolbe.com/drag.htm
From your second link, summed it up nicely.
When the bullet velocity is less than 1600 fps, the G1 drag function just does not characterize the aerodynamic drag on the bullet. This causes the BC values to vary widely as the bullet velocity falls through the speed of sound (about 1120 fps) and to lower subsonic velocities. The step change method of adjusting BC values is, at best, a crude approximation. This situation is mitigated somewhat by the fact that aerodynamic drag on a bullet diminishes dramatically in the lower transonic and subsonic velocity regions. Consequently, the effect of large ballistic coefficient errors on bullet trajectories is much less than when the bullet velocities are above 1600 fps. For handgun bullet trajectories, the effect is also lessened by the fact that ranges to the targets or the game animals are considerably shorter than for rifles. But at the present time, accurate long-range trajectories simply cannot be calculated for bullets that travel at lower transonic and subsonic velocities. This affects the ballistics of rifle cartridges such as the 30-30 Winchester, 35 Remington, 444 Marlin, 45-70, and the “Whisper” class of cartridges, as well as all handgun cartridges chambered in rifles.”
 

spife7980

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Feb 10, 2017
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#18
So if this is what I am understanding,
While shooting subs, on short yardages.

I may get a better bc from round nose flat base bullets.
I may also get better groups?

I had never considered this possibility.
Thanks!
Not at all what the thread is about but no.
For subs the round nose will be short than a pointy bullet of equivalent weight. Shorter is easier to stabilize.
Accuracy is a separate matter as tiro said.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#19
I was/am looking at subsonic ammo stability comparing 7 vs 8 twist on a sierra 220 round nose for the new blackout Im building. I notice that on sierras site that a 220 round nose gains BC as it slows down according to their banded numbers which is the opposite of everything I had noticed until now. I then went and looked at a 150 gr flat base round nose and it gains BC as well. So I then compared to boattail versions of equivalent weights and the boat tail versions lose BC like I was accustomed to seeing. I thought it might be due to the roundnose vs pointed but it appears that its just a flat base vs boat tail phenomenon.

The op started with 300 blackout subs, then the transition talk came up.
I thought you came up with very valid info for the difference in the projectiles at the subsonic speed.

I had always thought pointy boat tails were the best , your findings say maybe not.
So I have to test some new stuff now.

Not sure you can get 220 in hv in a 300 blk case? Was confused by this.
Go ahead and set me strait, just not seen it.

Thank you for finding this!
 

spife7980

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Feb 10, 2017
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#20
I am the op. For best accuracy you may find that a light weight bullet at sub speeds is best in your gun. Wish the picture links were still good.
http://forum.snipershide.com/threads/ts-customs-223ai-300blk-switch-barrel.6475994/

But the longer the bullet is the further out it’s weight sits on its axis of rotation. Like a football, the tighter the spin the more likely it is to spiral in line, slow the spin down and it will wobble about like a winged duck. So the slower it goes the more it needs to spin to maintain its stability. Not really anything directly transition related.

So as far as transition, no, getting 220s fully supersonic isn’t feasible in a blackout. But I don’t think you would notice much difference in the little bit you can get above fully subsonic.
Now 22s, if competing at 100, yeah, you may have the best accuracy with something like eley match that’s subsonic. The high velocity 1200 fps stuff can go through transition issues before you get to the target so that can be a bad choice but it can also be just fine.

And not subsonic but some people who shoot 30br for benchrest choose to shoot 110 grain things at short range for best accuracy.

In my blackout for hunting I’ve taken to running 150 gr at 1800 fps but that’s just due to the poor terminal performance of subs, not a lack of accuracy.

I just wanted to see the difference in twist rates and that was just the stories foreplay to the real issue I wanted to discuss: why the bc grew, not what’s best in a blackout.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#21
A very good (point) lol, has been brought out by this for me.
The flat base bullets are going to get a second look.
I was in the boat tail camp and never looked at anything else.

I believe I read once that the flat base added extra drag and that actually aided stability in the subsonic range, (short range ) only probably, and mistakenly discounted it.

Thanks Spife!
 

bohem

PVA's HMFIC
Jan 6, 2009
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#22
Below is a rather simplistic explanation and I'm not putting equations in there but the conceptual approach is as follows:


A very good (point) lol, has been brought out by this for me.
The flat base bullets are going to get a second look.
I was in the boat tail camp and never looked at anything else.

I believe I read once that the flat base added extra drag and that actually aided stability in the subsonic range, (short range ) only probably, and mistakenly discounted it.

Thanks Spife!
Short Range BR only shoots flat based bullets. The flat base takes a step out of the forming process and leaves less bullet in the expansion region where the flow around the bullet is more turbulent. This is why they tend to be more precise.


Nope. Something is wrong here. BC can’t go up if velocity goes down. They are directly related.
Below the near sonic realm (Critical Velocity) the BC increases substantially because the shockwave is not being generated around the bullet anymore.
Then as you approach low subsonic speeds (below the compressible region) the BC falls again as the boundary layer on the bullet grows.

The data is pretty clearly demonstrated by the Cd vs. Velocity curves that are all over the internet. Most of them show the drag curve reamining dead flat down below appx M0.9 and at a magnitude that's only a few % of what it was at M2+ but if you look at radar data you'll see that drag curve increase as it drops out of the compressible region.

One ballistician's book even points this out in most radar data he had available. He incorrectly proposes that it's catastrophic tumbling causing it but the behavior is shown in bullets known to transit the sound barrier and fly accurately, point forward, for hundreds of yards past the transition zone.


I have looked at a bunch of radar data for my flatline designs and seen the same behavior as they get below M0.85-M0.9 as well. Once they drop below critical velocity (see dfn below) then the BC jumps up and then comes down again.

Critical Velocity is the point where the flow around an object goes supersonic (Mach 1.0, M1) even though the object itself is not traveling at M1.0 or greater.
 
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