I just replaced the enormous gas block on my M&P15 flat top with a YHM low profile clamp on block. Doing some measuring I found the gas port on both blocks were the same distance from the rear of the blocks. The OE block sandwiched the forearm retainer bracket to the shoulder on the barrel. The bracket was .030" thick. I fashioned a spacer to go between the new block and the shoulder. This put the barrel port and block port in alignment. Without the spacer (or just leaving a .030" gap) the ports would be partially blocked. I have had no problems using PMC ammo in this rifle.