Steyr Mannlicher
The Mk 20 Mod 0 was a 40mm automatic grenade launcher manufactured by NOS Louisville which was used by the US Navy during the Vietnam War. It used a blow-forward mechanism that was previously used on semi-automatic pistols such as the Steyr Mannlicher M1894 and Schwarzlose Model 1908. The Mk 20 was eventually replaced by the Mk 19 Mod 3.