Twist rate and penetration with M193

[mkbar80]

If we are running 55gr/M193 in suburban/urban area, is a 1:7 or 1:9 more approriate if overpenetration is the primary concern? A slower twist rate would be sufficient for stabilizing the 55, but I have also recently read that in Stoner's orignial design a slower twist rate was used (in the original rifle length system) to impart a "just stable" flight on the round, resulting in significant cavitation and instability upon impact. In short....

Does anyone have any knowledge of how twist rate can effect penetration with the 55gr/M193 Round?

Would be run on a 16" Mid-length system

[lanceriley]

if you can find the original 1:12 that would probably be a good barrel. I've tried that in an 11.5in it was a cut down from an m16 barrel. Shot small game (same size as a dog of 20kgs) exit wound was 3in

[bkb0000]

so long as the projectile is spinning on its axis when it hits, i dont think it's going to make a difference. if anything, a fast twist rate seems more likely to fragment on impact, which is what youre looking for.

would be a good question for the terminal ballistics section

[kmrtnsn]

If over penetration is the concern, why are you running FMJ?

[mkbar80]

Originally Posted by kmrtnsn

If over penetration is the concern, why are you running FMJ?

My fault...I indicated 55gr/M193 just as example of the mass of round (your correct, but for some reason M193 just came to mind. Mouth was ahead of brain). Plink/Practice with reloaded 55FMJ, but have been happy with Hornady TAP/VMax bullets. The only pentration experience I have with the VMax is shooting it through some scrap plywood/drywall from 25 and 50 meters (and was suprised by how much pentetration I still got). It seemed I didn't get much penetration afterwards, if I got a partial keyhole on the second or third layer, the times without any keyholing the fragments pentrated a noticable bit farther...but I only had enough material to shoot a few rounds at each distance. The keyhole made me think of tumbling, tumbling made me think of twist rate...but I'm far from an expert.

Guys, I appreciate the input, I hadn't thought of the rate of spin leading to more fragmentation. That is an interesting point that I hadn't thought of.

[tpd223]

According to what I have read twist rate doesn't matter except where accuracy is concerned.

[Iraqgunz]

Were you shooting the TAP ammo through a 1/9 twist by the way? If so, it's not surprising that there was some keyholing because 1/9 is generally not suited for those heavier loads. At least not consistently anyways.

Originally Posted by mkbar80

My fault...I indicated 55gr/M193 just as example of the mass of round (your correct, but for some reason M193 just came to mind. Mouth was ahead of brain). Plink/Practice with reloaded 55FMJ, but have been happy with Hornady TAP/VMax bullets. The only pentration experience I have with the VMax is shooting it through some scrap plywood/drywall from 25 and 50 meters (and was suprised by how much pentetration I still got). It seemed I didn't get much penetration afterwards, if I got a partial keyhole on the second or third layer, the times without any keyholing the fragments pentrated a noticable bit farther...but I only had enough material to shoot a few rounds at each distance. The keyhole made me think of tumbling, tumbling made me think of twist rate...but I'm far from an expert.

Guys, I appreciate the input, I hadn't thought of the rate of spin leading to more fragmentation. That is an interesting point that I hadn't thought of.

[Cameron]

Originally Posted by Iraqgunz

Were you shooting the TAP ammo through a 1/9 twist by the way? If so, it's not surprising that there was some keyholing because 1/9 is generally not suited for those heavier loads. At least not consistently anyways.

Couldn't he be shooting the light(er) TAP loads? 40, 55, 60 or 62gr?

[Iraqgunz]

Yes, he could. But it seems from what I remember it is the top end heavier loads that seem to be problematic. He shouldn't have issues with 60 or 62. No clue about 40, but 55 should be Ok as well.

Originally Posted by Cameron

Couldn't he be shooting the light(er) TAP loads? 40, 55, 60 or 62gr?

[Ga Shooter]

Thanks

[mkbar80]

Thanks to bkb for pointing me to Doc's terminal ballistics page....as you can tell from my post count I don't know my way around here much yet. Input here is awesome, especially for someone with out a lot of technical experience with the AR system.

Yes on currently using a 1:9, and have been happy with accuracy out to 150M with up to 62gr green tips. The rifle is more accurate than I am anyway, even with the heavier loads. I've never shot anything heavier than 62 out of the 1:9. I'm upgrading a RRA with either a new upper or swapping a new chrome barrel (twist rate?), BCG, etc. I also have an old Colt Sporter that could use some updating. One will become a legitimate defensive carbine, the other will likely wait and be a plinker or become an updated rifle length system.

Anyone else with info on how they believe twist rate can effect penetration, all else being equal, I would appreciate the info. I'm going to follow up with a question on the terminal ballistic page.

[Failure2Stop]

From the data I have seen the difference in twist rate alone with compatable ammunition is insignificant.

If over-penetration is your primary concern you should probably be looking in a direction other than firearms as your solution. 12-14 inches of penetration in gelatin is considered to be the best option as it permits destruction of vital organs even when engaged at an oblique or laterally. Unfortunately, many people are thinner from chest to back than 14", which means that you are going to have to rule out the most effective ammunition out of fear of over-penetration.

To negate over-penetration issues think about altering the angle of attack through positioning to prevent the bullet from exiting in a direction you don't want it to go. Selecting the same load as local/state police is also a good option as far as liability is concerned (or at least so I am told).

[Molon]

Originally Posted by mkbar80

Does anyone have any knowledge of how twist rate can effect penetration with the 55gr/M193 Round?

It doesn't, period. The original AR-15s had a 1:14" twist rate simply because that was the common twist rate of that era for use with 55 grain bullets in the caliber. However, the 55 grain boat-tail bullet used in M193 is longer than the typical flat base bullets used in that era. Testing in cold weather showed the 1:14" twist insufficient to adequately stabilize the 55 grain boat-tail bullet, so the twist was changed to a 1:12" twist rate.

The original twist selection had nothing to do with terminal ballistic considerations nor did the change to the 1:12" twist. The change to the 1:7" twist with the introduction of the A2 and SAW was to stabilize the M856 tracer bullet which is incredibly long and has a very low specific gravity and again, the change in twist rate had nothing to do with terminal ballistic considerations.

[CaptainDooley]

Originally Posted by Molon

It doesn't, period. The original AR-15s had a 1:14" twist rate simply because that was the common twist rate of that era for use with 55 grain bullets in the caliber. However, the 55 grain boat-tail bullet used in M193 is longer than the typical flat base bullets used in that era. Testing in cold weather showed the 1:14" twist insufficient to adequately stabilize the 55 grain boat-tail bullet, so the twist was changed to a 1:12" twist rate.

The original twist selection had nothing to do with terminal ballistic considerations nor did the change to the 1:12" twist. The change to the 1:7" twist with the introduction of the A2 and SAW was to stabilize the M856 tracer bullet which is incredibly long and has a very low specific gravity and again, the change in twist rate had nothing to do with terminal ballistic considerations.

I am by no means trying to argue with you as you seem to know a lot more about this than I do... However, the reasons they chose to change twist rates is to stabilize the bullet, but does that necissarily mean that it doesn't affect terminal ballistics? Is it possible that twist rates could affect terminal ballistics (seriously, I don't know), even though the choice to change twist rates was made for a different reason?

[lanceriley]

twist rate does matter with regards to penetration.

you have a m855 ammo in 1/12 twist. what happens? it is less stable / totally unstable.

for the bullet to start tumbling, it has to be destabilized. if your bullet starts to tumble you have less penetration.

you need just enough stability to put the ammo on target. once it has penetrated the target, we want the bullet to "destabilize" to produce the tumbling effect.

so stabilize your bullet enough to reach it's mark.

[Failure2Stop]

Rifling twist rate is generally the minimum amount to stabilize a bullet through temperature extremes in air. Human tissue is something like 400 times more dense than air, so the difference between 1:7 to 1:9 in tissue is like the difference between 1:0.0175 and 1:0.0225 in air, which is to say that both are completely unstable.

[lanceriley]

I don't think so.. if there is little difference with air and tissue. 1:12 won't tumble in tissue.

[Failure2Stop]

Bullet construction, length, and angle of impact will have far more to do with terminal effect than twist rate.

The twist rate affects nothing of consequence outside of the gyroscopic stability needed to keep the bullet stable during it's time between the muzzle and target. What happens when the bullet hits tissue is much more similar to fluid dynamics than aerodynamics.

There is a great deal of difference between air and tisse (as I said, something like 400 times as dense), in fact there is so much difference that it is implausable to construct a barrel with sufficient twist rate to keep the bullet stable through tissue.

If there is a need to limit penetration, the answer lies in bullet construction, not twist rate, except to provide the correct level of stabilization to the projectile through air to ensure sufficient precision levels.

ETA- this is probably more suited for the terminal ballistics or ammunition forum.

[lanceriley]

we don't need the bullet to be stable while within tissue. we want it to be unstable resulting in more tissue damage. yes it will penetrate and exit in pieces...

[KevinB]

Originally Posted by lanceriley

I don't think so.. if there is little difference with air and tissue. 1:12 won't tumble in tissue.

WTF



Heed what F2S wrote

FYI Tumble is a poor word - Yaw is a better word for what BT bullets do in tissue.