These weapons began to be developed before the I G.M. but they were extended during the II G.M. they are based on the expulsion of part of the powder gases through the cylinder head, to compensate for the recoil force. The North American M-20 gun appeared in 1945, operating in the Pacific, where it was highly appreciated for its lightness; used, the tripod of the 7.62 mm Browning M1919 machine gun. Being widely used during the Korean War. Although it proved ineffective against the T-34 tank, it was used primarily to attack all types of field fortifications and light armored vehicles. They arrived in Spain after the Spanish-American aid agreements of 1953.