The Walther Colt M4 Carbine is a replica of the M-16 machine gun used by military; whereas, the Walther Colt M4 OPS replicates the M-16 machine gun used by US special forces. The .22LR Colt M4 Carbine features a carbine length barrel, collapsible stock, removable carry handle, and a high quality aluminum upper and lower receiver. The .22LR Colt M4 OPS features an OPS length barrel, collapsible stock, quad skeleton forend, accessory rails, and high quality aluminum upper and lower receiver.