The M16A2 buttstock is about five/8" of an inch for a longer time then the earlier styles. As a result of increased length it applied an extended buttstock screw and a spacer. The swivel is mounted closer into the buttplate. This buttstock is at this time applied within the M16A4: https://gold-cup-trophy-colt-4546666.bluxeblog.com/63383754/the-2-minute-rule-for-colt-combat-unit-rail-gun-45