USED AMERICAN TACTICAL - ATI MODEL GMB3R, SERIAL NUMBER AT23003341, 12 GAUGE, 18.5 Notes: The American Tactical MB3-R (also known as the ATI S-Beam MB3-R) is an affordable 12-gauge pump-action shotgun primarily designed for home defense. It is widely recognized as a 1:1 clone of the Remington 870, meaning most 870-pattern parts and accessories are compatible, Gauge: 12 Gauge. Action: Pump-action with a smooth bore barrel. Capacity: 4+1 rounds. Barrel Length: 18.5 inches. Chamber: 3-inch (accepts 2- 3/4" or 3" shells).