Barrel Twist vs. Fleet Yaw

AR/M4 AR Technical Discussion
1

Fleet yaw/Barrel twist…

I have heard that 1/7 will cause M193 and to a lesser extent, M855 to “overstabalize” (whatever that is…) and “drill” through a target without tumbling/fragging.

However, I would THINK that spinning the round faster would cause more fleet yaw because any imperfection in the core/jacket would be magnified at a higher rpm. (causing the projectile to more readily tumble as the larger degree of yaw would cause the side of the nose of the bullet impacting first to slow down before the other side, inducing tumble much sooner than a 0* yaw impact).

Thus 1/7 would technically be more lethal than a 1/9 twist or slower, unless the twist did not stabalize the bullet and thus induce a LOT of fleet yaw or even…dare I say it…tumbling in flight.

Thoughts/test data/field reports?

Just something I was curious about and wondering if anyone who has “been there-done that” had an opinion/fact to share?

2

M855, in general, sucks for anything past 400 yards for precision engagement. Modern production equipment just can’t place perfect steel cores coaxially with bullet boreline with the jacket and lead core.

Spin a bullet too fast (especially one with an off-center core, inducing asymmetric “Wobble”) and you get bigger groups and flyers. If the spin is fast enough the bullet’s jacket won’t stay together and you’ll often see a grey cloud somewhere between 10 feet and 50-100 yards downrange as the bullet spins itself apart.

We see lots of this at Camp Perry with people shooting thin J4-jacketed bullets. They have a tendency to sacrifice velocity for accuracy and the bullet gets overspun. The rifling engraving (especially from a quality, cut-rifled match barrel) tends to weaken the jacket and the over-spin and centrifugal force takes care of the rest.

You see this mostly in the 600-yard slow-fire stages of the National Match Course (shooting 80-grain Noslers out of ARs and M16s) and the 1,000-yard matches (shooting 6.5 X 284s).

3

WS6,

It seems you might be confusing a few issues, especially with regard to Fleet Yaw vs. AOA variations, as well as the relation of twist rate and wounding effects.

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.

As Sinister notes above, 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. Sinister hit the nail on the head in discussing this issue in his post above.

Angle-of-Attack (AOA) variations between different projectiles, even within the same lot of ammo, as well as Fleet Yaw variations between different rifles, were recently elucidated by the JSWB-IPT. These yaw issues were most noticeable at close ranges (usually under 25 m) and were more prevalent with certain calibers and bullet styles.

Projectile impact angle-of-Attack (AOA) variability is caused by bullet to bullet variations at impact and can substantially alter wound severity; this factor is more prevalent with certain calibers and projectile types. Testing demonstrated that 5.56 mm is highly susceptible to AOA variations, particularly when using FMJ projectiles 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 other calibers were less susceptible to AOA variations than 5.56 mm, OTM’s tend to have less AOA issues than FMJ, while PT and JSP’s tend to not have major AOA induced terminal performance changes.

Fleet Yaw is the terminal performance AOA variation caused by inherent variability in each rifle; Fleet Yaw is caused by weapon to weapon variations separate from projectile induced AOA issues.

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 opponents are not being incapacitated because M855 is zipping right through the targets without upsetting. Both shooters are telling the truth… Restricting M855 for what is was designed for–use in the SAW and simply adopting new 5.56 mm barrier blind combat loads that are optimized for carbines with shorter barrels, offer consistent early upset, along with adequate penetration, and minimal AOA/Fleet yaw issues may be the critical answer to many deficiencies noted with currently issued U.S. military 5.56 mm ammunition.

4

THANKS! +1 to both of these answers. I “get it” now.

5

more info about bullets & twists here:

http://razoreye.net/mirror/ammo-oracle/AR15_com_Ammo_Oracle_Mirror.htm

:wink:

6

The bottom line ballisticians try to find as far as optimal twist and velocity – try to use the slowest twist and minimum charge to hit your bullet’s sweet spot (whether that’s a 5.56, 30 cal, 4.2-inch or 120mm mortar, 155 howitzer, or 16-inch naval rifle).

Too fast a twist will exaggerate your spin dispersion. Try shooting the 500- or 600-yard National Match Course prone bullseye target with a random draw of M855 Green Tip with a standard M16A2 or M16A4. Don’t be surprised if you can hold an “X” and it will come up as a non-visible miss. Optimize your combination and know your dispersion and drift and you can drop a 155 projo into a trash can at 25 kilometers or from a 105 sticking out the aft port side of an AC-130.

Try shooting M855 out of a 1-12 M16A1 Carbine and you’ll be lucky to get a random broadside keyhole impact at 25 yards.

Settle on the optimum twist for what you’ll draw at work or buy at the store.

1-7, 1-8, and 1-9 seem to work well from 55 to 77.

Most shooters will never shoot past 100 yards in precision application anyway. If you’re shooting IPSCs or E-types you should be able to hit with any of the three. At 25 Meters it makes absolutely no difference.

7

Ok, so how do the Russian bullets get two 180 flips of the bullet in test media and ours only get one?

8

Because our bullets fragment after the 1st yaw.

9

A better question would be how this is physically possible. The whole reason a projectile flips is to put the “weight foreward”. Why would it DE-stabalize after the first flip to put the weight behind? That makes 0 sense to me. Someone educate me.

10

Some bullets even have 3 lobes to the yaw cycle…Now, if you look carefully at the drawings, for the first lobe of the yaw cycle, the projectile yaws to near 90 degrees, but does NOT flip over to 180 degrees. Once the bullet fully yaws past 90 degrees in tissue, it does NOT generally return to point forward travel.

11

Once a tail-heavy bullet starts to yaw (the heavy ass end wants to trade ends and go base-forward once it’s hit something semi-solid) if it’s spinning fast enough to come apart it will. If it’s solidly built or has a thick, solid jacket (like Russian or 30 cal bullets) the damn thing just starts going end-over-end – it won’t spin itself apart and disintegrate.

US 30 call and 7.62 ball are solidly-built, thick-jacket bullets without air spaces. They will tend to deform but stay together.

12

Sinister is right on, the vast majority of U.S. .30 caliber service ammunition has one yaw cycle in tissue and no fragmentation. We have previously tested a variety of historically significant .30 cal U.S. service rifle FMJ ammunition, including: original pre-WWI .30-06 loading (1912 manufacture date), pre-WWII .30-06 M1 ball, WWII .30-06 M2 ball from 1942, .30-06 M72 Match, the original 7.62 x 51 mm M59 FMJ load which preceeded the current 7.62 x 51 mm M80 ball load, and 7.62 x 51 mm M118 SB (Match). All these FMJ bullets travel point forward for 6" or so, causing minimal tissue damage. The bullet would then yaw, turning 180 degrees and continuing base forward without further yaw. In uncomplicated extremity wounds, the bullet will frequently exit before yawing, causing little tissue disruption and small punctate entrance and exit wounds. If the bullet yaws while still traveling in tissue, in the thick torso for example, the permanent and temporary cavity are increased while the bullet travels sideways, resulting in a greater amount of crushed tissue and extensive damage to inelastic tissue. Exit wounds may be punctate, oblong, or stellate depending on the bullet yaw angle on exit.

As mentioned before, the JSWB-IPT recently discovered that Lake City has manufactured TWO distinct types of M80 FMJ over the last several decades. The version tested by Dr. Fackler at LAIR uses a thick copper jacket that does not fragment and acts like the older .30 caliber projectiles discussed above; the other previously untested version uses a steel jacket that fragments at velocities above approximately 2800 f/s. Yaw and fragmentation occurs at about the same penetration depth as with copper jacket M80 and is thus deeper than ideal (ie. NL too deep). When the velocity drops to below 2800 f/s (by approx 100 m from 22" barrel M14), M80 steel jacketed bullets no longer fragment and instead act like the non-fragmenting copper jacketed M80 FMJ. Note that LC does NOT distinctly label the different M80 FMJ projectiles and the only way to tell them apart is to use a magnet.

As noted in previous posts, both the 7.62 x 51 mm M852 and M118LR use SMK (168 gr & 175 gr) OTM bullets which may fragment during yaw, although their terminal performance is extremely inconsistent, as discussed by both Fackler and Haag. If fragmentation does not occur, they act like the FMJ loads described above. When the SMK bullets do fragment, the bullet typically travels approximately 5" point forward before beginning to deform, yaw, flatten, and fragment. Approximately 63% of the bullet’s weight is lost to the multiple fragments which spread radially outward from the primary bullet path. The multiply perforated tissue is unable to resist the stretch induced by temporary cavitation and extensive tissue destruction occurs. Uncomplicated extremity and torso wounds would likely exhibit a small punctate entrance wound. If the bullet exits before deforming or yawing, minimal tissue disruption will occur and a small punctate exit wound will be present. After the bullet yaws, deforms, and fragments, tissue destruction is greatly increased; torso wounds are often fatal and exit wounds may exhibit large tissue defects.