Early Gen SIG MCX. 5.56 NATO. Has a SIG factory gen bolt upgrade with spring. Rifle length pencil barrel. Key mod rails. Comes with several picatinny rails, front qd sling, rear SIG qd adapter plate. This is the hybrid model that's been discontinued by SIG. It's a SIG AR lower with an MCX adapter so any AR lower can be used with the adapter for MCX uppers. Runs great and low round count. I just don't use it after getting a shotgun.