The weapon was developed together with other similar devices early in 1917 and went into service in June 1917 in response to German unrestricted submarine warfare. It was mounted on merchant ships and patrol vessels. By 10 December that year, 377 were in service.[1]
The shell was fired at the submarine either on the surface or submerged - hence it had attributes of both armour-piercing shell and depth charge. It was designed to first penetrate the submarine's outer hull without breaking up, and then detonate against the inner hull after a 2-second delay,[2] destroying the submarine. However the small weight of the shell limited its effectiveness,[1] and the anti-submarine mortar did not become a truly successful weapon until the advent of the multi-barrelled Hedgehog during World War II.
Nonetheless Admiral John Jellicoe, writing a few years after the gun had gone into service, commented: "This weapon, although not very popular at first, soon, however, proved its value, when employed both from patrol craft and from merchant ships."[1]
Two were mounted on HMS Vindictive and used to bombard German defences during her participation in the Zeebrugge raid of 23 April 1918.
