Boys Anti-tank Cartridge .55". It has a band at base of casing and marked on base "K.41 W.II" K = Kynoch 41 = 1941; w = Armour Piercing; II = Mark II The Bullet is 735 gr. and 50 mm long. The official designation is: Cartridge, SA, Armour-Piercing. .55 inch, Mark II. The Rifle, Anti-Tank, .55in, Boys commonly known as the "Boys Anti-tank Rifle" (or incorrectly "Boyes"), was a British anti-tank rifle in use during World War II. It was often nicknamed the "elephant gun" by its users due to its size and large bore.[3] There were three main versions of the Boys, an early model (Mark I) which had a circular muzzle brake and T-shaped monopod, built primarily at BSA in England, a later model (Mk I*) built primarily at Jonathan Inglis in Toronto Canada, that had a square muzzle brake and a V shaped bipod, and a third model made for airborne forces with a 30-inch (762 mm) barrel and no muzzle brake. There were also different cartridges, with a later version offering better penetration. Although adequate against light tanks in the early part of the war, the Boys was ineffective against heavier armour and was phased out in favour of the PIAT mid-war. Not counting experimental rounds, there were thirteen different loadings of the .55 Boys approved for service: Ball, Mark 1 Ball, Mark 2 Ball, Mark 3 Practice P Mark 1 Practice P Mark 2 Armour-Piercing W Mark 1 Armour-Piercing W Mark 2 Tracer G Mark 1 Proof Q Mark 1 Proof Q Mark 2 Proof Q Mark 3z Drill D Mark 1 Dummy U Mark 1