An Accuracy Comparison of M855 When Fired From 1:9” Twist and 1:7” Twist Barrels.

AR/M4 AR General Discussion
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An Accuracy Comparison of M855 When Fired From 1:9” Twist and 1:7” Twist Barrels.

This test will compare the accuracy (technically, precision) of M855, produced by Israel Military Industries (IMI), when fired from an AR-15 with a chrome-lined, NATO chambered barrel with a 1:9” twist and an AR-15 with a chrome-lined, NATO chambered barrel with a 1:7” twist. The IMI M855 is the singularly most accurate and consistent loading of all the M855 clones that I have tested to date.

IMI M855 is loaded with a 62 grain FMJ bullet with a cannelure. It also has a steel penetrator seated in the ogive portion of the projectile. This makes the projectile unusually long for its weight, as well as giving it a lower specific gravity. The ammunition is charged with “ball powder”. The primers are sealed and crimped in placed. The bullet is also crimped in place and the case mouth is sealed with an asphalt sealant.

Chronograph data for the IMI M855 was obtained using an Oehler 35P with “proof screen” technology. All velocities listed are muzzle velocities as calculated from instrumental velocities using Oehler’s Ballistic Explorer software program. All strings of fire consisted of 10 shots each.

This accuracy evaluation used statistically significant shot-group sizes and every single shot in a fired group was included in the measurements. There was absolutely no use of any group reduction techniques (e.g. fliers, target movement, Butterfly Shots).

The shooting set-up will be described in detail below. As many of the significant variables as was practicable were controlled for. Also, control-groups were fired from the barrels used in the evaluation using match-grade, hand-loaded ammunition; in order to demonstrate the capability of the barrels.

All shooting was conducted from a concrete bench-rest from a distance of 100 yards (confirmed with a laser rangefinder.) The barrels used in the evaluation were free-floated. The free-float handguard of the rifle rested in a Sinclair Windage Benchrest with the aid of a Sinclair fore-arm adaptor, while the stock of the rifle rested in a Protektor bunny-ear rear bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Wind conditions on the shooting range were continuously monitored using a Wind Probe. All rounds were fired from the magazine. Each upper was fired using the same lower. The set-up was very similar to that pictured below.

The barrels used in this evaluation were both Colt HBARs with NATO chambers and chrome lining. The 1:7” twist barrel was 20” in length and the 1:9” twist barrel was 16” in length. When fired from the 16” barrel with a 1:9” twist, the M855 projectile will have a gyroscopic-stability-factor of approximately 1.24 and when fired from the 20” barrel with a 1:7” twist the gyroscopic-stability-factor will be approximately 2.07. I purposely selected the shorter barrel with a 1:9" twist and the longer barrel with the 1:7" twist in order to exacerbate any possible statistically significant influence that the differing twist rates might have on the precision of the M855 ammunition.

Colt 16" HBAR

Colt 20" HBAR

Prior to the firing of the M855, control groups were fired from the test barrels using match-grade handloads seated with Sierra 55 grain BlitzKings. The 10-shot control group fired from the 1:7” twist barrel had an extreme spread of 1.17” and the 10-shot control group fired from the 1:9” twist barrel had an extreme spread of 0.89”.

Three 10-shot groups were fired from the upper with the 1:9” twist barrel in the manner described above. The extreme spreads for those groups measured:

2.72”
2.19”
2.24”

for a 10-shot group average of 2.38”. The three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius for the composite group was 0.76”.

In the same manner as above, three 10-shot groups were fired from the upper with the 1:7” twist barrel. The extreme spreads of those groups measured:

2.14”
3.01”
1.71”

for a 10-shot group average of 2.29”. As before, the three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius for this composite group was 0.68”.

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A very interesting and informative post as usual. Thanks for taking the time to do all the work and share it with us here.

3

Thanks for posting the results.

4

Super informative post. Thanks for the info!

5

What, if any, factors would the differing barrel lengths have on the results?

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Great job! Thank You for your efforts and posting your findings.

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Basically it was a wash with this lot of IMI ammo. I’m curious what a 16" with a 1:7" Colt HBAR would have achieved with the same lot. Or even a 16" with a 1:7" Govt. M4 profile barrel to compare with these results using M855. Here’s a link to Molon’s accuracy test of a 14.5" SOCOM. Also shows results of two 16" M-4 profile barrels. Though match grade ammo was used in the testing.

https://www.m4carbine.net/showthread.php?59364-The-Colt-M4A1-SOCOM-Barrel

Thanks Molon for continuing to keep M4Carbine.net on course with informative content. :thank_you2:

Some more data to peruse from Molon.

https://www.m4carbine.net/showthread.php?61263-Accuracy-Evaluation-of-Six-Different-Noveske-Barrels

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Nice! Thank You

9

How is this “a wash”? The point of this test was to show that people who keep making excuses for their subpar barrels dont have a leg to stand on. There is absolutely zero point to having a 1:9 twist barrel in a fighting gun.

10

If they are never going to shoot heavy bullets, who cares?

11

Because these people can see the future right?

12

Well then, I have quite a few things I need to change in my life.

13

This is good information Molon. Your differences in muzzle velocity based on barrel length mirror what I have seen during testing.

Have you done this comparison of twist rates with M193 and the same rifles you used in this test?

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I would like to see those results also.

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By definition, an “unstable” bullet will have a gyroscopic stability factor of less than 1.0 at the muzzle. A typical 55 grain FMJ bullet will have a gyroscopic stability factor of approximately 3.7 when fired from a 20” barrel with a 1:7” twist. [CaptainObvious] 3.7 is not less than 1.0. [/CaptainObvious].

The following demonstration compares the results of firing four 10-shot groups of the same lot of 55 grain Prvi Partizan M193 ammunition from two different barrels; a chrome-lined, NATO chambered barrel with a 1:9” twist, and a chrome-lined, NATO chambered barrel with a 1:7” twist. The first barrel used in testing was 16” Colt HBAR with chrome lining, a NATO chamber and a 1:9” twist. The second barrel used in testing was a 20” Colt HBAR, also with chrome lining, a NATO chamber and of course a 1:7” twist. The longer barrel with the 1:7” twist was purposely chosen for the increased muzzle velocity coupled with the 1:7” twist. Both barrels had free-float handguards.

Accuracy (technically, precision) testing was conducted from a distance of 100 yards following my usual protocol. The fore-ends of the weapons rested in a Sinclair Windage Benchrest and the butt-stock rode in a Protektor rear-bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Naturally, the wind conditions were monitored using a Wind Probe. The set-up was very similar to that pictured below.

Four 10-shot groups of the PPU M193 were fired from the 1:9” twist barrel. Those groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius of that composite group was 1.08”.

As with the 1:9” twist, four 10-shot groups were fired from the 1:7” twist barrel. Those groups were also also over-layed on each other to form a 40-shot composite group; the results were nearly identical to those obtained from the 1:9” twist barrel. The composite group had a mean radius of 1.01”. The two composite groups are shown side by side for comparison.

Testing performed by C.E. Harris at Aberdeen Proving Ground and later at Sturm-Ruger has shown that the “overstabilization” claim is largely nonsense. The testing showed that “overspinning” quality light-weight bullets from a fast twist barrel does not become an issue unless you have a gyroscopic stability factor greater than 5.0 (which would require something along the lines of a 1:6” twist barrel launching a 55 grain bullet at over 3500 fps) or unless firing at an angle greater than 85 degrees.

After the U.S. Military adopted the 1:7” twist for the M16A2, C.E. Harris did extensive testing comparing the accuracy of light-weight bullets fired from 1:10” twist barrels and 1:7” twist barrels using 52 grain Sierra MatchKings. The accuracy testing was done from 200 yards, (well into the downward slope of the trajectory where the Internet Commando claims that all manner of evil befalls the “overstabilzied” bullet), and the accuracy results from the two different twist barrels were also nearly identical.

Quality, modern lightweight bullets (40 to 55 grain) of typical copper-jacket/lead-core construction can shoot superbly from fast twist barrels. Typical 55 FMJ bullets do not fall into the quality category.

The group pictured below was fired from a 1:7" twist Noveske barrel using 40 grain V-MAX bullets.

The 10-shot group pictured below was fired from a distance of 100 yards from a Noveske barreled AR-15 with a 1:7” twist.

The 3-shot group pictured below was fired from the same 1:7” twist barrel for the Internet Commandos in our viewing audience.

The next 10-shot group pictured was fired from a Krieger barreled AR-15 with a 1:7.7” twist using 55 grain BlitzKings.

Here’s another example of just how well a 1:7" twist barrel can shoot light weight bullets. While the group pictured below was fired from a distance of only 50 yards, it’s a 10-shot group fired from a chrome lined, NATO chambered Colt barrel with a 1:7" twist using 52 grain Sierra MatchKings.

A 14.5" Colt M4A1 SOCOM barrel,with its NATO chamber, chrome lining and 1:7" twist can shoot quality 55 grain bullets quite well.

A 1:7” twist rate does not have a negative effect on the terminal ballistic properties of M193 and M855. From Dr. G.K. Roberts . . .

“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.”

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Unreal…your posts are ridiculously enjoyable. Thanks for another ace.

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Awesome post, very interesting. You answer those questions that I talk to my buddies and I tend to only speculate about. Thanks for the interesting read.

18

The post that keeps getting better. Thanks Molon. :slight_smile:

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Thanks for posting. It would be interesting to see a similar test done at say 200-400 yards with an extrapolation of the 100 yard data to see if it holds up.

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I think it shows just the opposite. That there is no real loss of accuracy with a 1:9 twist barrel and the M855 ammunition.

An increase in 0.08" (the thickness of a comic book) in the mean radius over 100 yards is unnoticeable if you are using a 2 MOA red dot, and, if you were using iron sights, this increase would be lost in your aiming errors.

As to the 16" vs 20", other tests (http://www.thetruthaboutguns.com/2013/10/daniel-zimmerman/the-truth-about-barrel-length-muzzle-velocity-and-accuracy/) have shown there is little loss of accuracy when going from a longer to a shorter barrel, less than .1" on average at 100 yards on average and almost zero for 18" to 16" barrels.