Barrel Twist in relation to Bullet Yaw

[PA PATRIOT]

I have done some reading on how a barrels twist rate can affect the yaw of a bullet after impact due to the bullet being under stabilized in flight. Now the one thing I cant find is how much of a affect a given twist rate may have on the popular 5.56 bullet weights from a standard 16" thur 20" barrel. Some say that a 1-12"inch twist is better for the 55gr to 62 grain bullets to cause earlier yaw while others claim a 1-14" twist. Now I understand there will be a affect on accuracy using these slower twist rates but if the engagement ranges were short less then 100yds what could one except? Would it still be CQC accurate.

[Zhukov]

The formula formula to determine the gyroscopic stability of a bullet is given by:

http://www.nennstiel-ruprecht.de/bullfly/gyrocond.gif

So an increase in w by going to a faster twist rate in the barrel is more than offset by the thousandfold increase in p once the bullet encounters tissue. In other words: Tissue is so much more dense than air that the slightly faster rate of bullet rotation is negligible; the bullet will be completely unstable as soon as it encounters tissue.

[PA PATRIOT]

I need a more "Layman's" reply if you would sir.

Thank you.
Phila PD

[Molon]

I need a more "Layman's" reply if you would sir.

Thank you.
Phila PD

As long as the bullet is adequately stabilized at the muzzle, barrel twist rate has no effect on the terminal ballistic properties of 5.56mm ammunition.

[rubber ducky]

Don't know how helpful this will be, but I've fired M855 through what i think it was a 1:14 (it could have been a 1:12) 16" bbl and it keyholed at 100m.

[tpe187]

As originally designed by Eugene Stoner, the M16 had a 1/14" twist. This kept the 55gr bullet at the ragged edge of stability. When the bullet impacted flesh it rapidly yawed and fragmented. In military testing by the Air Force in arctic conditions, it was determined that accuracy became unacceptable at 65 below and a change to 1/12 twist was recommended to further stabilize the round for arctic conditions - just in time for the jungles of Vietnam. "Misfire", pg 490.

Some critics contend that this single change resulted in the M16 being 40% less lethal with the change from 1/14 to 1/12 twist. "The Great Rifle Controversy" pg 199

The 1/7 twist of the M16A2 was in response to the length of the M856 tracer round, originally developed for the M249 SAW.

Unfortunately, most of the civilian examples of the AR15 use a 1/7 twist because that is what the military uses, though it is not the most appropriate twist.

[larry0071]

Ok, admitted dum-dum sticking his nose in here...... sorry....

Since I run basically 100 yards and less and only 55 gr XM193 ammo, would my 1:9 barrel be improved by being a 1:12 or 1:14? That is what I am hearing.

I don't ever plan to be shooting out past 100 yards because the property I shoot at does not readily provide me with that type of distance. I'm also not into the various military ammo types and I again buy what is readily available.... Federal XM193 black box or Wolf 55 grain .223 spec ammo.

It sounds as if the SHTF performance would be better with the 1:12 or 1:14 style twist, but that brings forth a new question... who makes a chrome lined, 16" or 14.5" carbine barrel with those slow twist rates?

[PA PATRIOT]

As originally designed by Eugene Stoner, the M16 had a 1/14" twist. This kept the 55gr bullet at the ragged edge of stability. When the bullet impacted flesh it rapidly yawed and fragmented. In military testing by the Air Force in arctic conditions, it was determined that accuracy became unacceptable at 65 below and a change to 1/12 twist was recommended to further stabilize the round for arctic conditions - just in time for the jungles of Vietnam. "Misfire", pg 490.

Some critics contend that this single change resulted in the M16 being 40% less lethal with the change from 1/14 to 1/12 twist. "The Great Rifle Controversy" pg 199

The 1/7 twist of the M16A2 was in response to the length of the M856 tracer round, originally developed for the M249 SAW.

Unfortunately, most of the civilian examples of the AR15 use a 1/7 twist because that is what the military uses, though it is not the most appropriate twist.

So if the 1-14 twist rate was considered "Ideal" by Stoner for the 55gr bullet could the same be said for the 1-12 twist rate some claim is ideal for the 62gr version. Does anyone have a link to testing which proves or disproves this assumption?

[Shawn Dodson]

From "Review of Infantry Magazine 2006 Lethality Article" by Doc Roberts:

P29. “The IPT was ultimately able to determine a reason for the differences.”
While this first sentence is true, the remainder of this paragraph is not. The apparent differences in 5.56 mm performance were obvious on viewing high speed video of the projectiles’ flight paths from muzzle to impact and noting the differences in yaw behavior. Discovering this had NOTHING to do with the ARL “dynamic” methodology which uses the flawed computer simulated “virtual human target” (ie. a naked man with his hands at his sides standing directly 90 degrees frontal to the shooter).

As touched upon by MAJ’s Dean and LaFontaine on p31, Angle-of-Attack (AOA) variability at impact can substantially affect wound severity; this factor is more prevalent with certain calibers and projectile types. JSWB-IPT testing demonstrated that 5.56 mm projectiles are highly susceptible to AOA variations, particularly when using full metal jacket (FMJ) loads such as M193 & M855. For example, with 5.56 mm FMJ, at higher AOA’s, for example 2-3 degrees, bullets had a shorter neck length (NL) and upset rapidly, thus providing adequate terminal effects; at low AOA, like 0-1 degree, the projectiles penetrated deeper than ideal prior to initial upset (ie. long NL) with significantly reduced terminal effects. Note that OTM’s were less susceptible to AOA variations than FMJ. Other calibers were less susceptible to AOA variations than 5.56 mm; the 6.8 mm proved to have less AOA inconsistencies compared with other calibers tested.

Fleet Yaw is the other significant yaw issue discovered by the JSWB-IPT. Fleet Yaw is the terminal performance variation caused by inherent variability in each rifle and occurs in all calibers. 5.56 mm FMJ appears to suffer more Fleet Yaw induced variability than other projectile calibers & types. 6.8 mm OTM’s appear to have less Fleet Yaw variations than other projectile calibers & types tested.

What this means is that two shooters firing the same lot of M855 from their M4’s with identical shot placement can have dramatically different terminal performance results: one shooter states that his M855 is working great and is effective at dropping bad guys, while the other complains his opponent is not being incapacitated because M855 is zipping right through the target without upsetting. Both shooters are telling the truth…

-- https://www.m4carbine.net/showpost.php?p=393730&postcount=1

[Heavy Metal]

Zuhkov and Molon gave you your answers, ya'll are giving weight to information and data that is obselete and long discredited. The math Zuhkov posted does not lie.

If you shoot a .55gr projectile into a block of gelatin or a live animal, asumming everything else is equal, you will not be able to tell whether it was fired from a 1/7 or a 1/14 twist barrel.

The original 1/14 twist was not a deliberate attempt by Stoner to do anything.

It is simply that .224 cal barrel blanks available in the late 50's/early 60's were almost all 1/14.

This is what he used.

The Black Rifle book is 25 years old and far from authoratative in light of what we now understand.

[Heavy Metal]

So if the 1-14 twist rate was considered "Ideal" by Stoner for the 55gr bullet could the same be said for the 1-12 twist rate some claim is ideal for the 62gr version. Does anyone have a link to testing which proves or disproves this assumption?

A 1/12 twist will not even stabilize the long 62gr SS109 projectile found in M855 ammo.

This renders it far from ideal.

[DocGKR]

Please...

The U.S. Army Wound Ballistic Research Laboratory conducted terminal performance testing using 5.56 mm 55 gr M193 FMJ ammunition fired in 20” barrels of 1/14, 1/12, 1/9, and 1/7 twist rates. No difference in terminal performance was noted between shots made with the different twists. Similar testing was conducted with 5.56 mm 62 gr M855 FMJ ammunition fired in 1/9 and 1/7 twist barrels. Again, no difference in terminal performance was noted. There are some projectiles where the terminal performance can be effected by twist rate, but these are not generally in military use. Also, if the bullet is not adequately stabilized in flight, then alterations in the wound profile will be evident.

Twist rate can definitely effect external ballistics. For example, in testing ammunition at the CHP Academy in the mid 1990’s, a number of lightweight, thin-jacketed, relatively high velocity .223 varmint loads were observed to disintegrate in mid-air a few yards from the muzzle when fired from fast 1/7 twist weapons, but not in slower twists; the Federal 40 gr Blitz loading was particularly problematic in this regard. Likewise, long 70+ gr projectiles don't always stabilize in 1/9 or slower twist barrels.

Personally, I prefer 1/8 or 1/7 twist for 5.56 mm weapons.

[rubber ducky]

Likewise, long 70+ gr projectiles don't always stabilize in 1/9 or slower twist barrels.


I was going to add... with my limited knowledge regarding twist rates and bullet stabilization - my understanding is that its really not the bullet weight, but rather bullet length... So, if you shoot same weight bullets of different lengths, you will get different results - assuming you're using the same barrel.

My apologies if this contributed nothing to this thread.

Sincerely,

rd

[K.L. Davis]

Yep, it is really the the length of the projectile...

1:8 twist will spinup anything that you can fit in a magazine and works for nearly all conditions and ranges a person could ever encounter -- benchrest shooting is a world where a person can learn and obsess about all of the minutia... but rest assured that 99.9% of the time, 1:8 RoT (or there abouts) is taking care of everything for you, no user input needed

[rubber ducky]

... 99.9% of the time, 1:8 RoT (or there abouts) is taking care of everything for you, no user input needed

if I recall correctly, I think the "magic number" was something along the lines of 1:7.8

[dewatters]

As originally designed by Eugene Stoner, the M16 had a 1/14" twist. This kept the 55gr bullet at the ragged edge of stability. When the bullet impacted flesh it rapidly yawed and fragmented. In military testing by the Air Force in arctic conditions, it was determined that accuracy became unacceptable at 65 below and a change to 1/12 twist was recommended to further stabilize the round for arctic conditions - just in time for the jungles of Vietnam. "Misfire", pg 490.


The USAF report that led to the adoption of the 1:12" twist, "Exterior Ballistics of the AR-15 Rifle", is available online at DTIC. One thing to note is that the research was performed under the supervision of Gerald A. Gustafson. One of the early SCHV advocates at Aberdeen, Gustafson transferred to the USAF Armament Center in the mid-'50s after Dr. Carten refused to fund further SCHV cartridge development by Aberdeen's Small Arms and Aircraft Weapons Branch.*

As someone else mentioned 1:14" twist was commonplace for other rifle cartridges using 0.223-0.224" projectiles. However, these cartridges were typically loaded with lighter and shorter projectiles than the 55gr FMJ-BT used in the .223 Remington. The comment about -65F is a smokescreen. While they did test to temperatures that low, stability of the 55gr FMJ-BT projectile from 1:14" twist barrels had already gone to pot by 0F. They indicated that the 1:14" twist would be adequate for a flat-base 55gr FMJ, but this would reduce the maximum range of the projectile.**

As the USAF was the primary user of the AR-15/M16 in 1963, their needs were considered paramount. The USAF has bases in Alaska and other nasty cold places. Major deployments to South Vietnam of US Army combat units with the XM16E1 did not occur until 1965. In the meantime, McNamara refused to allow additional AR-15/M16 to be issued to the South Vietnamese after the ARPA trials. Mass issue of the M16A1 to regular South Vietnamese military units did not begin until after McNamara left office in 1968.

When the rifling twist controversy reared its head again in 1967, a large batch of 1:14" barrels (~1,000) were made and assembled into rifles for comparison testing against an equal number of rifles with 1:12" barrels. In the summer of 1967, Colt testing found that the 1:14" barrels shot groups twice as large as the 1:12" barrels. Colt lobbied against the reversion in rifling twist as before the transition to 1:12" twist barrels, they had to reject 10% of the 1:14" twist barrels in acceptance testing due to insufficient accuracy.

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*On a side note, the USAF's Dale M. Davis was assigned to Aberdeen and worked under Gustafson during the SCHV experiments. Davis returned to the USAF Armament Center around the same time Gustafson transferred. Davis was later responsible for the creation of the "Arm Pistol" concept which led to the Colt IMP.

**Ironically, Remington's 55gr FMJ-BT design was shorter than the original Stoner/Sierra projectile design. The BRL at Aberdeen later found that if they wanted to revert to the Stoner/Sierra projectile, the proper rifling twist would have been around 1:10". As they had already replaced the 1:14" barrels, the military wasn't going to turn around and then scrap all of the 1:12" barrels.

[benthughes]

The formula formula to determine the gyroscopic stability of a bullet is given by:

http://www.nennstiel-ruprecht.de/bullfly/gyrocond.gif

So an increase in w by going to a faster twist rate in the barrel is more than offset by the thousandfold increase in p once the bullet encounters tissue. In other words: Tissue is so much more dense than air that the slightly faster rate of bullet rotation is negligible; the bullet will be completely unstable as soon as it encounters tissue.

Ummmm....Duh!

[DocGKR]

There is ongoing new research indicating that with some expanding projectiles, especially in shorter barrels, FASTER twists produce improved terminal ballistic performance...

[Ernst]

Ummmm....Duh!


Exactly, I mean, even a pre-schooler knows this stuff, right?

[vicious_cb]

There is ongoing new research indicating that with some expanding projectiles, especially in shorter barrels, FASTER twists produce improved terminal ballistic performance...

Not sure how I missed this gem of information. Any updates on this phenomenon? Any theory on why this occurs?

[ColtSeavers]

Exactly, I mean, even a pre-schooler knows this stuff, right?

09-14-09, 20:12...

[butlers]

As long as the bullet is adequately stabilized at the muzzle, barrel twist rate has no effect on the terminal ballistic properties of 5.56mm ammunition.

I apologize for resurrecting this thread, but I'm researching a retro A1 build and I wanted to understand a bit more about the 1:12 twist rate (vs. the "modern" 1:7 twist rate).

From the information shared here, it seems that the following statement is wrong?


"This is important if you're shooting living targets, e.g., hunting because the fragments, combined with hydrostatic shock, tend to produce a much larger and more potentially lethal wound cavity than a bullet that stays in one piece. The relevance to rifling twist is that usually only a very rapidly spinning bullet - say, over 250,000 rpm - will fragment like that. A 1:12 twist won't produce that effect"
https://www.at3tactical.com/blogs/news/14625981-ar-15-barrels-barrel-length-gas-systems-and-rifling-part-2

So, 55gr XM193 out of a 14.5" barrel with 1:12 twist rifling goes around ~3000 fps and is spinning at 180,000 rpm according to this RPM calculator:
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

Same bullet weight, same barrel length, but 1:7 twist rifling and slightly less velocity (~2900 fps) produces ~300,000 rpm.

The difference in RPM only effects the flight path of the bullet, and not the lethality -- right?

[T2C]

As long as the bullet is adequately stabilized at the muzzle, barrel twist rate has no effect on the terminal ballistic properties of 5.56mm ammunition.

I believe this statement sums up the affect twist rate has on terminal ballistics.

[Todd.K]

For practical understanding that statement still holds true. The simple answer is "barrier blind expanding" and then go train.

I have seen some discussions about higher RPM causing slightly more damage with some bullet designs, I believe fragmenting.

[vicious_cb]

For practical understanding that statement still holds true. The simple answer is "barrier blind expanding" and then go train.

I have seen some discussions about higher RPM causing slightly more damage with some bullet designs, I believe fragmenting.

Another reason to go 1/7 albeit a small one.


There is ongoing new research indicating that with some expanding projectiles, especially in shorter barrels, FASTER twists produce improved terminal ballistic performance...