M4 quick recognition guide: 14.5 inch "M-203" barrel (the indentations are for mounting the M-203 Grenade Launcher), retractable multi-position stock (evolved from the XM-177/CAR-15), 3-position fire selector, available in flat top or fixed carrying handle versions, may or may not have any of the various Picatinny rail systems or a standard forearm.
