You should shoot no heavier bullets than 55 grain 223/5.56 ammunition through it! Any bullet heavier than 55 gr. will have difficulty stabilizing in flight which will severely effect accuracy. If you shoot 69 grain or heavier you will probably experience side trikes on the target or key hole strikes at 100 yards. That is because the 1:12 twist is not fast enough to stabilize the heavier bullets. I posted a thread in the sticky section of the AR-15 Forum regarding barrel twist if you get a chance to look at it!