I searched the forum but didn't find anything so I figured I'd throw this up here. Thought this was pretty cool. Stumbled across it browsing the web today. From what I gather from the reading, it's a drop in upper for any AR lower. Found it at http://www.brpguns.com/categories/Semi-Auto-Firearms/XMG-Belt%252dFed-AR%252d15%7B47%7DM16-Upper-Receiver/ I've noticed they are sold out of the 308 version.