The M16A2 Buttstock is about five/eight" of an inch for a longer time then the sooner models. As a result of increased length it applied an extended buttstock screw as well as a spacer. The swivel is mounted nearer on the buttplate. This buttstock is presently used within the M16A4: https://franciscogjihi.shivawiki.com/6966011/what_does_colt_combat_unit_rail_gun_45_mean