Davis gun

The Davis gun was the first true recoilless gun developed and taken into service. It was developed by Commander Cleland Davis of the US Navy, in 1910, just prior to World War I.

Back to Back
His design connected two guns back to back, with the backwards-facing gun loaded with lead balls and grease of the same weight as the shell in the other gun, acting as a counter. His idea was used experimentally by the British and America as an anti-Zeppelin and anti-submarine weapon mounted on the British Handley Page O/100 bomber and the American Curtiss HS-2L and H-16 (flying boats)  respectively. The direct development of the gun ended with World War I, but the firing principle has been copied by later designs.

Statistics
The gun was made in three sizes of 1.57, 2.45 and 3'', taking 2, 6 and 12 pound shots respectively. The 3'' carried a pressure 15 tons per square inch when fired. Usually a Lewis machine gun was mounted on top of the Davis gun's barrel which was then used for sighting and as an auxiliary and anti-aircraft weapon.

Museum Pieces
There is an example still at the Naval Aviation Museum at Pensacola, Florida and another at the Imperial War Museum in London.