Both of my AR's have 1-7 twist barrels and I shoot 55gr bullets all the time with no issues and good accuracy. In fact, the 20 inch heavy barrel one, I have shot 52gr SMK bullets with sub moa groups. They actually grouped better than the 75gr hornady bullets I have tried. In theory, the 1-7 should have done better with the heavier bullets, but it did not. So it goes to show how each barrel/gun can be different.