Many people, who ask me about building their own AR-15, always seem unsure about which barrel twist to get for their rifles. Hopefully this mini article will help you guys figure out what is best for your needs.
In general, rifling twist rate determines the optimum weight of the bullet for a given caliber. It also determines the speed of the bullet by preventing any yaw or pitching. Rifling twist is measured in the number of revolutions per inch of barrel. For instance a 1 in 9″ twist means that the bullet made one revolution while traveling 9″ down the barrel.
So, what is the best twist rate? Well the answer nobody wanted is, “It Depends!” 🙂 A good rule of thumb is that the more weight or longer the bullet is, the faster the twist rate has to be in order to stabilize the bullet. Also, in general, lighter/shorter bullets can usually be fired in barrels with faster twist rates, but heavier/longer bullets cannot be fired in barrels where the twist rate will be too slow. Let’s examine this in more deatail as it relates to AR-15’s.
The original M-16 started off with a 1 in 14″ twist rate which is good enough for bullets around 55gr. However, when temperatures dipped below freezing, the density of the air caused the bullets to lose their spin, resulting in much less accuracy. In order to solve this problem, the military adopted a twist rate of 1 in 12″. The SS109/M855, which is 62gr, required a 1 in 10″ rate to stabilize, but the military settled on a 1 in 7″ rate due to the need to fire the heavy and long tracer rounds.
What does this mean for the civilian AR-15 enthusiast? Well, considering most barrel manufacturers produce their barrels in both 1 in 9″ and 1 in 7″, choosing either will not hurt the performance of the rifle for the majority of shooters out there. If you think you will be shooting the heavier grain bullets or tracer rounds, then it is probably safer to pick a 1 in 7″ twist rate barrel.
I hope that this short article on AR-15 barrel twist rates helped clear up some information! Please feel free to leave us some comments or questions if you would like more information!