M3 Grease Gun Optic MountOptic mount for the M3 Submachine Gun (Grease Gun). Easy to install. Rail is made of aluminum and mount is made of impact resistant plastic. Simple one screw and rare earth magnets hold mount in place. Does not require modification to the firearm and can be removed easily to use the existing iron sights. The M3 is an American . 45 caliber submachine gun adopted by the U. S. Army on 12 December 1942, as the United States Submachine Gun, Cal. . 45, M3.