This article will be looking at twist rates and bullet weight for AR15 type rifles. Though the theory can be passed on to most rifle makes and calibre's. In these modern times, bullets for military and civilian use have migrated to longer, heavier designs, and twist rates have been altered to suit these new advances.
The rifling in your barrel imparts a spin onto the projectile, which stabilises the bullet during flight and prevents excessive yaw and tumble. In the AR15 type rifle, slower twist rates have issues stabilising heavier bullets. If we look at the M16A1 rifle (used by American troops in Vietnam), it was designed with a 1:12inch twist, meaning that for every 12inches of barrel, the rifling does one complete revolution. This is a slower twist rate than more modern AR15's, and will produce a slower spinning projectile during flight. This rifle was designed to shoot the 55-grain military 5.56mm ammo. On the other hand, using modern military AR15's with 1:7inch twists can cause lighter bullets to over-stabilise, causing similar problems.
A large enough imbalance in bullet weight to twist rate can cause poor accuracy, decreased velocity, and can weaken the bullets structural integrity.
Many people these days prefer carbine length barrels. These shorter barrels were designed to put troops in and out of vehicles and into close-quarter fighting. These shorter barrels require heavier bullets to compensate for lost velocity.
1:12inch Twist - this older design suits lighter bullets like the 55-grain FMJ. You could push the weight up to 60-grain, but going to heavy will produce far less accuracy.
1:9inch Twist - this is the common twist in South Africa for most semi-auto Ar15's on the market. This twist is the perfect compromise, and allows the shooter to take advantage of the heavier bullet, which will produce far greater stopping power in the case of the .223 calibre. The rifling is not too fast to cause problems with the 55-grain bullets, but fast enough to stabilise the heavier bullets under most circumstances. You may start to experience issues when using the heaviest 77-grain bullets with shorter barrels.
1:7inch Twist - this twist is chosen for the modern military AR15's. It is designed to shoot heavier bullets, which require a fast twist and produce advanced external and terminal ballistics.
Matching your rifling twist to the appropriate ammunition will not guarantee accuracy, but it will ensure the bullet is properly stabilised during flight. Experiment with different bullet weights, aiming to achieve accurate groupings on target, and see which one best suits your rifle. Our Advanced Sniper manual teaches you skills and techniques to achieve the greatest accuracy, and gives you the tools to become an expert sharpshooter.
