In the AR-15 the most common twist rate is 1 in 9". This will stabilize up to 69 grains quite well. I've read that some get fairly good accuracy with up to 75 grains, but that's pushing it. for 70 grains and above a 1 in 7" twist is preferable, and even necessary. You can try some 75 grain ammo in a 1 in 9" twist and see how it performs. The worst is they will keyhole, but you'll only be out a box of ammo. For very light bullets from 45 to 55 grains a 1 in 12" twist will work. Many .22-250 barrels are rifled with a 1in12", or a 1 in 10" twist. The longer the bullet the more twist is required to stabilize it. Weight is a second consideration to length. Bill T.