Webly MK VI? Hmm, let me think. OK, 2 seconds past. GREAT! Love them. 6 inch barrel, 265 gr of lead pushing 700 fps! Ever read Churchill? Have you read about Rourke's Rift? The Martini-Henry gets most of the coverage, but the officers had Webleys (or their own purchase) in .476 ( very close to the .455).
The US Sam Colt in .44-40 and .45 Colt had similar ballistics. The Webley had the heft, double action, sinble action and ejected quickly for a fast reload. There was also a crafted "speed loader" available for those with enough savey to get them and use them.
The accuracy of good ammo fitted to the revolver was very good. I don't have a MK VI, but do have a MK I and MKIV. Both converted to .45 ACP or Auto Rim. I load for the same ballistics as the .455 or .476.
Search and find my Post
http://forums.nitroexpress.com/showflat....true#Post113209
If it is in good shape, .455 or .45 auto rim, you can't really go wrong.