Which Twist Rate is Best for Your AR-15
Testing done by: +Andrew Tuohy
One of the most-discussed topics regarding the AR-15 and similar .223/5.56mm rifles is rate of twist. A faster rate of twist will spin the projectile more over a given distance than a slower rate of twist, and rate of twist is closely related to projectile length and diameter.
This isn't really an issue with a lot of calibers, but because of the relatively narrow diameter of the .224" projectile, minor changes in weight or material can result in significant changes in length. If one chooses to shoot only the most common ammunition on the market, 55 grain lead core FMJ, the choice of twist rate is not essential - practically any .223 or 5.56mm barrel on the market will work with that bullet.
However, there is a wide variety of .223 ammunition available, and in times of short supply, sometimes one might have to buy ammunition that has longer or shorter projectiles than "normal." This could be anything from 35 grain varmint hunting ammo to 77 grain match grade ammo.
Many claim that if the twist rate is too fast, the bullet will become "overstabilized" and either come apart in midair or tumble out of control. The twist rate many blame for this condition is 1/7. If the twist rate is too slow, the bullet will yaw in midair and be less accurate than it would have otherwise been. At the extreme end, the projectile will "keyhole," or impact the target sideways. Firing 75 to 77 grain match bullets with 1/9 or 1/12 twist barrels can cause this.
While preparing for a potential LuckyGunner Labs post on the topic, I did some preliminary testing with ammunition representing basically everything on the market - from 35 to 77 grains. I fired this ammunition through 1/7, 1/9, and 1/12 twist barrels.
All groups were fired from the bench with a sandbag for support and various scopes, all of which were set at 9x magnification. Single 10 shot groups at 100 yards were fired on days with nearly identical atmospheric conditions (temperatures in the mid-80s with low humidity).
The 1/7 twist barrel was button rifled stainless steel, 16" in length, and chambered for .223 Wylde. The 1/9 twist barrel was hammer forged chrome moly steel, chrome lined, 16" in length, and with a 5.56mm chamber. The 1/12 twist barrel was hammer forged chrome moly steel, 20" in length, with a .223 Remington chamber.
The 1/7 and 1/9 barrels were installed in an AR-15 rifle with a free floated handguard. The 1/12 twist barrel was used in a bolt action rifle.
Again, this was preliminary testing and not the strictly controlled testing that we would normally use for an LGL post, but the results were still interesting. Due to the differences in barrel construction and action type, comparing group sizes directly is not representative of all barrels of that twist rate, but stability was what the test was really about.
The 1/7 twist barrel did not overstabilize any of the ammunition types. It was remarkably accurate with some of the lighter bullets. Although it generally shot better groups with the heavier projectiles, it was accurate and precise enough with the light bullets for almost any purpose, including varmint hunting.
The 1/9 twist barrel properly stabilized all of the lighter and medium weight bullets, but 77 grain projectiles were yawing significantly at 100 yards.
The 1/12 twist barrel shot very well with all of the lightweight bullets. It also properly stabilized 62 grain lead core projectiles, but with 62 grain steel penetrator M855 ammunition, it keyholed. The same went for 69gr and heavier ammunition - complete 10 shot groups were not fired for 75 and 77 as the first shots were keyholing.
It appears from this preliminary testing that the conventional wisdom regarding 1/7 twist and light .223 ammunition is at least partially incorrect. Even the 35 grain bullet traveling at nearly 3800 feet per second stayed together and proved to be remarkably accurate from the fast twist barrel.
If you'd like to see more testing on this topic at http://www.luckygunner.com/labs
- please let us know.