The A2X is an extended version of the A2 flash hider found on the M16 and M4 series of rifles. When mounted per BATF specifications for a permanent installation it will bring your 14.5 M4 barrel to a non-NFA length of over 16.1. Installation should be done by a qualified gunsmith familiar with BATF regulations. The A2X is predrilled and comes with a dowel pin and crush washer for installation.