cartridge in an anti-tank rifle, but Pershing insisted the cartridge be rimless. The first .50-caliber machine gun underwent trials on 15 October 1918. It fired at less than 500 rounds per minute, and the muzzle velocity was only . Cartridge improvements were promised. The gun was heavy, difficult to control, fired too slowly for the anti-personnel role, and was not powerful enough against armor. While the .50-caliber was being developed, some 13.2×92mmSR Mauser 1918 T-Gewehr anti-tank rifles and its ammunition were captured. The 13.2mm German rounds had a muzzle velocity of , an bullet, and could penetrate armor thick at a range of . Winchester improved the .50 caliber round to have similar performance. Ultimately, the muzzle velocity was . Efforts by Browning and Fred T. Moore resulted in the water-cooled, .50 caliber M1921 Browning machine gun and an aircraft version. These guns were used experimentally from 1921 until 1937. They had lightweight barrels and the ammunition fed only from the left side. Service trials raised doubts about whether the guns would be suitable for aircraft or for anti-aircraft use. A heavy barrel M1921 was considered for ground vehicles. John M. Browning died in 1926. Between 1927 and 1932, S.H. Green studied the design problems of the M1921 and the needs of the armed services. The result was a single receiver design that could be turned into seven types of .50 caliber machine guns by using different jackets, barrels, and other components. The new receiver allowed right or left side feed. In 1933, Colt manufactured several prototype Browning machine guns (including what would be known as the M1921A1 and M1921E2). With support from the Navy, Colt started manufacturing the M2 in 1933. FN Herstal (Fabrique Nationale) has manufactured the M2 machine gun since the 1930s. General Dynamics, U.S. Ordnance and Ohio Ordnance Works Inc. are other current manufacturers. A variant without a water jacket, but with a thicker-walled, air-cooled barrel was designated the M2 HB (HB for Heavy Barrel). The added mass and surface area of the heavy barrel compensated somewhat for the loss of water-cooling, while reducing bulk and weight: the M2 weighs with a water jacket, but the M2 HB weighs . Due to the long procedure for changing the barrel, an improved system was developed called QCB (quick change barrel). The lightweight "Army/Navy" prefixed AN/M2 "light-barrel" version of the Browning M2 weighing was also developed, and became