Hispano-Suiza 20mm Cannon

U.S. Navy. Via ibiblio.org

Hispano-Suiza HS 404

Specifications: 

The Hispano-Suiza cannon was designed by Marc Birkigt, a Swiss working in France, and was adopted by Britain's Royal Air Force as the Hispano-Suiza Type 404. It later saw service with the Americans as the AN-M2. Both the British and the Americans considered adapting the Hispano-Suiza as an antiaircraft gun, but chose the 20mm Oerlikon instead.

The Hispano-Suiza had a gas operated delayed blowback action. The bolt was locked to the breech until the projectile passed a gas port in the barrel, which operated a piston that unlocked the bolt. The bolt then recoiled under the residual pressure in the barrel, like any other blowback action. However, because the bolt was initially locked in place, it could be made much lighter than the bolt in an ordinary blowback action, permitting a much higher rate of fire. The reduced weight also made the weapon more suitable for mounting in aircraft.

This novel action was initially rather buggy and required that the cartridges be waxed for clean extraction. The British eliminated the need for waxed cartridges, apparently by using a fluted chamber that allowed a small amount of gas from the fired round to work around the outside of the cartridge and push it away from the chamber walls. The British also found that the action was violent enough to compress the cartridge as it was fed into the chamber, leaving the primer far enough forward that the firing pin occasionally failed to activate the primer. This problem was solved by shortening the chamber length by about 2 mm (0.08"). Improvements were also made to the reliability of the magazine and feed mechanism.

The weapon was designed to be mounted in a hollow propeller shaft, which led to some peculiarities in design. It was extremely long and slim, and it did not come with a fixed receiver, being intended to be mounted directly to the V-block of an inline engine. This required a very sturdy structure when the gun was mounted in wings or turrets, which the British developed as the SAMM cradle. The weapon could not be synchronized to fire through a propeller.

The gun was heavy and was originally fed from a 60-round drum. Smaller drums were devised for flexible mountings, and a 160-round drum was designed for fixed guns, though this was unreliable if loaded with more than about 150 rounds. The somewhat low rate of fire was compensated by an unusually powerful cartridge.

The British initially produced the Type 404 as the Mark I. This proved unsatisfactory due to the small magazine capacity and a tendency for the gun to jam during high-G maneuvers. The Mark II was fed from a disintegrating link belt and was less prone to jam, and it saw extensive use. The Mark V was also belt-fed and had a shorter barrel that could be entirely contained in a fighter wing. This avoided the mechanical stress and freezing problems arising from the protruding barrel in Mark II installations, at some cost in muzzle velocity (about 80 fps or 24 m/s).

The British were anxious for American manufacturers to produce the weapon, which was expected to be in high demand. However, the Americans failed to adopt the British modifications, continuing to use waxed cartridges and failing to shorten the chamber to avoid misfires. It became evident that the AN-M1 had so high a misfire rate that it was unusable, but only after 56,410 had already been manufactured. The AN-M2 shortened the chamber by 1mm and made some other improvements for reliability, but American-manufactured Hispano-Suizas remained sufficiently unreliable that the gun came to be disliked by most American pilots. The gun averaged one jam every 1500 rounds under good operating conditions, but twice that rate under dusty conditions. The gun was more vulnerable than the Browning to cold temperatures at high altitude. Those installed on the SB2C were particularly notorious for jams, although the problems were largely worked out in the SB2C-4 and may have been due as much to poor maintenance as anything else in the earlier SB2Cs.

Part of the continuing difficulty with the weapon was bureaucratic in origin. Under U.S. Army regulations, a weapon of over 0.60" (15mm) bore was considered artillery, and so the Hispano-Suiza was manufactured to artillery tolerances. This made for badly fitting parts, a fault that was long concealed by the practice of putting a heavy coat of lubricant on the cartridges. Postwar, the weapon became highly reliable, suggesting that there was nothing fundamentally wrong with the weapon. It was simply rushed into production before it had been completely debugged.

Some 134,663 AN-M1 and AN-M2 cannon were manufactured in the United States. Almost none of the M1 and only a fraction of the M2 produced were ever mounted in aircraft, and production ceased in February 1944. No use was ever found for the AN-M1s, but the Navy developed an improved lightweight version of the AN-M2, the T31, which like the British Mark V used a shortened barrel. The T31 could be converted from existing AN-M2 cannon, and some 32,346 were so modified by May 1945, when conversion ceased. New production of the T31 (not from conversion of the AN-M2) became the AN-M3, and this designation is used by most authors for conversions as well. Over 90% of 20mm cannon mounted in U.S. aircraft were mounted in U.S. Navy aircraft. 

Video of modern Hispano 20mm cannon being fired

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References

Chinn (1951; accessed 2014-3-20)

Tillman (1997)

Williams and Gustin (2003)

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