Rifle, Anti-Tank, .55in, Boys commonly known as the 'Boys (or, often and incorrectly,'Boyes') Anti-tank Rifle' was a British anti-tank rifle. There were two main types, an early model (MK I) which had a circular muzzle brake and T shaped bipod, and a later model (Mk II) that had a square muzzle brake and a V shaped bipod. There were also different cartridges, with a later one offering better penetration.