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:



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.p...30&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.