Recoilless guns

Military inventors were long attracted by the prospect of abolishing recoil, since achieving this meant doing away with the gun's heavy recoil system and lightening the carriage. The first to succeed was Commander Cleland Davis of the U.S. Navy, who in 1912 developed a gun with a single chamber and two opposite barrels. One barrel carried the projectile, the other an equal weight of grease and lead shot. The explosion of the central cartridge ejected both loads, and, since the recoils had the same weight and velocity, they canceled each other out and the gun remained stationary. Davis' idea was adopted in 1915 by the Royal Naval Air Service, which ordered guns of 40, 57, and 75 millimetres for arming aircraft against airships and submarines. Few were made, however, and there appears to be no record of their use in combat.

If the Davis principle were taken to its logical ends, the countershot could be half the weight and twice the velocity of the principal projectile or any other combination giving the same momentum; at its ultimate, the countershot could simply be a cloud of high-velocity gas. This was the system upon which recoilless guns of up to 105 millimetres were developed during World War II. The cartridge cases of these weapons had a weakened section that ruptured on firing, allowing about four-fifths of the propellant gas to be exhausted to the rear of the gun. There they passed through a venturi, a nozzle with a constricted portion that increased the gas velocity and so balanced the recoil generated by the projectile. The back-blast caused by the exhausted gases made these weapons difficult to emplace and conceal, but after 1945 they were universally adopted as light antitank weapons.