The M16A2 buttstock is about five/eight" of the inch for a longer period then the earlier styles. Due to enhanced length it used a longer buttstock screw and also a spacer. The swivel is mounted closer to your buttplate. This buttstock is at present used over the M16A4: I believed https://lorenzowxwtv.ampedpages.com/colt-m16a1-for-sale-things-to-know-before-you-buy-58442793