This is the famous Bofors, which was the most widely used AA gun of the Second World War. 34,116 were built in the U.S. during WW2, and they served in all theaters in most of the same types of units as the M1A2. A light-weight, air transportable version known as the M5, which when disassembled could be carried in a C-47, was also built. In order to supply both the U.S. Army and U.S. Navy with much greater numbers of the guns, Chrysler built 60,000 of the guns and 120,000 barrels through the war, at half the original projected cost, and filling the Army's needs by 1943. Over the lifetime of the production, their engineers introduced numerous additional changes to improve mass production, eventually reducing the overall time needed to build a gun by half; most of these changes were in production methods rather than the design of the gun itself. York Safe & Lock also produced the weapons, though its attempts to coordinate drawings across the program were unsuccessful, and this responsibility was transferred to the Naval Gun Factory in July 1943.
Bofors also developed a towable carriage which they displayed in April 1935 at a show in Belgium. This mount allowed the gun to be fired from the carriage with no setup required, although with limited accuracy. If time was available for setup, the gunners used the tow-bar and muzzle lock as levers, raising the wheels off the ground and thereby lowering the gun onto supporting pads. Two additional legs folded out to the sides, and the platform was then leveled with hand cranks. The entire setup process could be completed in under a minute.
The gun fired a 900 g high explosive 40 × 311R (rimmed) shell at 900 m/s. The rate of fire was normally about 120 rounds per minute (2.0 rounds per second), which improved slightly when the barrels were closer to the horizon as gravity assisted the feeding from the top-mounted magazine. In practice firing rates were closer to 80–100 rpm (1.3–1.7 rounds per second), as the rounds were fed into the breech from four round clips which had to be replaced by hand. The maximum attainable ceiling was 7,200 m, but the practical maximum was about 3,800 m. The gun was provided with an advanced sighting system. The trainer and layer were both provided with reflector sights for aiming, while a third crew-member standing behind them "adjusted" for lead using a simple mechanical computer. Power for the sights was supplied from a 6V battery.
The first version of the 40 mm the Navy ordered was intended for use on submarines, where the larger calibre allowed the gun to be used for both AA and against smaller ships. The barrel was shorter at 42 calibers long, with the effect of reducing the muzzle velocity to about 700 m/s. When not in use, the gun was pointed directly up and retracted into a watertight cylinder. The only known submarines that used this arrangement was the Sjölejonet-class boats. The guns were later removed as the subs were modified with streamlined conning towers.
Although originally intended as an Airborne weapon it was mainly used in the PTO—presumably by the 101st, 102nd, and 843rd AAA Automatic Weapons Battalions (Air-Transportable). Eighteen U.S. Bofors were field-mounted on M3 halftracks by the 209th AAA Automatic Weapons Battalion (SP), and in 1945 were used by that unit in the ground support role on Luzon.
