The .455 Webley MKII is a British revolver cartridge designed for their military in the late 1800's. This brass will work for all of the Webley MKII, MKIII,MKIV, MKV, and MKVI cartridges. The original MKI case is longer, but this will work as a short substitute for it.
It is important to note that the Webley cartridges designations don't necessarily coincide with that model revolver, and smokeless powder loads may not be safe in all revolvers.
*It is important to note that many Webley revolvers in the US have been modified to use the much thicker rimmed .45 Auto Rim cartridge by machining the cylinder to accept the thicker rims. Firearms modified as such will not work with standard .455 MKII brass.
Information sourced from https://www.starlinebrass.com/455-webley-mkii
