.223 Barrel Twist vs. Accuracy
By Chad Haire
Does standard 55 gr. ammo perform well in fast 1-to-9 twist barrels? Our M-16 Tells the story...
For several decades, the 5.56mm NATO (.223) cartridge consisted of a 55 gr. projectile fired out of barrels with a slow 1-in-12 inch twist. Lately, in an attempt to gain more range, the trend has been to switch to heavier 62 gr. projectiles. To stabilize the heavier bullets, rifles have been issued barrels with a quicker 1-in-9 or 1-in-7 inch twist.
While the 1-in-12 inch barrel can’t stabilize the heavier 62 gr. bullet, we have been told the 1-in-9 inch tube will work quite well with either the newer 62 or older 55 gr. projectiles.
The most popular rifle to find out with is the AR-15/M-16 series. Several years ago, Colt discontinued their old style A1 rifles which had the 1-in-12 twist barrels, and created the A2 series with the faster 1-in-9 tube. Afterwards, the national gun publications kept telling us how accurate the new models were, even when firing the lighter 55 gr. cartridges. Problem is, this does not agree with numerous AR-15/M-16 shooters I have talked to, who claim excellent accuracy with heavy .223 bullets, but lousy groups when shooting lighter projectiles out of their newer 1-in-9 barrels.
THE TEST
To find out what was true, and what was hype, I decided to conduct my own test. I acquired the following rifles:
1. Colt AR-15 SP-1 semi-auto rifle with 20-inch 1-in-12 barrel.
2. Colt M-16 select-fire with 20-inch 1-in-12 inch barrel.
3. Colt M-16 A2 series Carbine with 16-inch 1-in-9 barrel.
For the performance and accuracy evaluation, we purchased the following .223 ammunition:
1. PMC 55 gr. ball
2. IMI Samson 55 gr. hollow points
3. Winchester Super-X 55 gr. soft points
4. PMC 64 gr. soft points
5. JSC Barnaul (Russian) 62 gr. ball
For best accuracy results, all shooting was conducted within military spec “sight in” guidelines. This involved placing a 250 meter simulator target 25 yards away. This procedure eliminates light distortion and windage problems, which were very present in the hot Arizona desert we were shooting in. The 5-shot groups (I consider that three shot groups are not precise enough) would be multiplied by 4 to get the equivalent 100 yard group, or minute of angle (MOA).
TEST #1
First we fired the AR-15 with 20-inch 1-in-12 inch barrel. Our PMC 55 gr. ball showed a 3.4 inch MOA. Second, the Winchester 55 gr. S.P. did 2.5 MOA. Third, IMI H.P. gave a 5-shot group reading 2.5 MOA. This is a average of 2.8 MOA overall.
Next, we conducted the same firing sequence with our M-16 rifle. We got 4.5 MOA with PMC ball, which this particular rifle never liked anyway. Then 3.0 with Winchester S.P., and 3.0 using IMI H.P. rounds. The overall reading was 3.5 MOA.
Finally, the Colt Carbine with 1-in-9 barrel was fired. It showed 4.4 MOA with PMC ball, 4.0 for Winchester S.P., and 5.0 with IMI H.P. Overall average was 4.5 MOA.
I know what some of you might be thinking. Since both the 1-in-12 inch rifles had the standard 20-inch barrel, and the Carbine had a shorter 16-inch tube, the latter should not be as accurate as the longer former. At distances beyond 300 yards, yes there would be a difference due to a slightly slower bullet velocity. But at 25 yards? No way. Besides, the CAR-15 had a heavy bull barrel, so if anything, it should have been more accurate. The only significant difference (aside from the barrel twist, of course) that should be of concern is the front sight post design. The early A1 rifles have a thin round post, while the later A2 has a squared post that is much thicker, which hampers precise accuracy. To find out how much, I replaced the open sights with a 6-power scope, and shot the Carbine test over. The PMC showed little change, but the S.P. and H.P. did slightly better, with an overall reading from 4.5 to 4.0 MOA. This was still far worse than the 3.1 average of the two A1 models. Time for some different ammo!
TEST #2
Since the heavier 62 and 64 gr. bullets will not stabilize in the 1-in-12 barrels, the only rifle used for this last test was the Colt A2 rifle with 1-in-9 tube.
First, we shot the PMC 64 gr. soft-point ammunition. What a difference! The initial firing sequence gave us an MOA reading of 3 MOA, and the next was a 2.5 MOA for 2.75 overall.
Next, on out list was the Russian 62 gr. ball made by the JSC Barnaul Machine and Tool Plant. This stuff came into the market just recently, and was selling for only $135 per thousand rounds as of May 1999. Is it any good, you ask? Seeing how we obtained consistent 2-inch MOA groups, with some bullets going through the same hole on the target, I would say very much so! JSC also makes 55 gr. 5.56mm ammo, but I could not find any for this test.
CONCLUSION
After I did this test, I discussed the results with many target shooters who use the AR-15 and M-16 system. The stories I heard were the same: Fast twist 1-in-9 and 1-in-7 barrels work fine with heavy bullets, but lose about 20-30% accuracy (their figures, not mine) when using the standard 55 gr. projectiles, which were designed for the slower 1-in-12 type. In spite of this many shooters use the lighter 55 gr. anyway because it’s much cheaper to buy, and more available. But if the 62 gr. JSC Russian ammo works as good in other rifles as it did in mine, and continues to sell this cheap, this should no longer be an issue. In any case, those who are not getting acceptable accuracy from their A2 “fast twist” barrels have a clear choice: use heavier bullets or change the barrel.
This article first appeared in Small Arms Review V2N11 (August 1999) |
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