7.92×57mm weapons were no longer considered useful by the "Luftwaffe" once enough MG 131s were available. The partial armour protection of most new military aircraft had caught up with the 7.92×57mm SmK cartridge by 1940. Many MG 15, MG 17 and more modern 7.92 mm MG 81 were then used by forces on the ground, especially since 1944. Many were modified with a bipod and simple metal stock, and other belt-fed MG 17 and MG 81 were built into dedicated anti-air machine gun twin and quad mounts.