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

[Molon]

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”.




….

[Eric]

Perhaps the 16" 1/9 was borrowed from a 6721? I lugged one around for many years.