The AR57 Is An Excellent High-capacity Yet Affordable Alternative For Self-defense and Varmint Hunting. The AR57 (also known as the AR Five Seven) upper receiver for the AR-15 has two claims to fame: a 50-shot capacity and downward ejection for ambidextrous operation. Operating by simple blowback, this upper is available in 6-inch pistol and 16-inch rifle versions, with the latter being reviewed here. Manufactured by the eponymous AR57 LLC, and chambered in 5.7x28mm, this upper is less powerful than the standard 5.56mm version, but it has certain tangible advantages, including reduced muzzle blast, a high practical rate of ï¬re, nonexistent...