The 1 means "1 rotation" and the number following it is how far, in inches, the bullet travels to complete that 1 rotation. So, a 1:7 would be 1 rotation in 7 inches. 1:9 would be 1 rotation in 9 inches. The idea here is that bullets of different length need a certain rotational speed to stabilize them. Since bullets aren't generally listed by length, and heavier bullets are generally longer and lighter bullets are generally shorter, most of the talk about barrel twist mentions bullet weight. The heavier (longer) bullets need a faster twist rate to stabilize them in flight. Guys shooting the lighter (shorter) bullets can use a slower twist barrel. Using a longer bullet in a barrel that's too slow a twist is going to give you a situation where the bullet will tumble as it flies, and that's not good. Using a shorter bullet in a barrel with too fast a twist and you can actually tear the bullet apart by overspinning it. Assuming the bullet stays together (it will unless you're trying to shoot real high velocity varmint loads out of a 1:7 or something similar) the disadvantage to overspinning a bullet is you see a slightly slower velocity, and the barrel will see a little more heat. For most general AR use, a 1:9 or 1:7 should do you just fine. If you're interested in shooting the 75gr and 77gr stuff you're probably safer getting a 1:7. Some people report good results with 1:9 shooting the 75 and 77 stuff, but it seems to be a crapshoot. IMHO the ideal barrel twist for an AR is 1:8, but they're not commonly found.
Last edited by CC556; 06-06-09 at 11:44
Reason: Forgot something.