The Mark VI was chambered for the .455 Webley round, a rimmed caliber first used in the mid to late 1880s. It fired a heavy (265-grain) lead bullet at moderate velocity; despite not being a "speed demon," it had a lot of punch and established a sterling record as a man-stopper in combat. Because .455 Webley ammunition was hard to find in the U.S., most of the post-war imported guns chambered for it were altered by the simple expedient of milling off the rear of the cylinder to increase headspace. This allowed use of the .45 ACP in special clips. In 1920, the Peters Cartridge Company brought out the ".45 Auto Rim," essentially a rimmed version of the .45 ACP, whose thick rim allowed it to be used in the Webley without special adapters. The .45 Auto Rim is a "kissing cousin" of the .455 Webley with ballistics similar to it and the .45 ACP.