I encourage folks to drop by here for some more info on the 455 rounds. No doubt one reason the Her Majesty's Empire collapsed was their cinfusing military ordnance nomenclature.
http://cartridgecollectors.org/?page=introduction-to-455-cartridges
http://britishmilitariaforums.yuku.com/topic/3053/455-Manstopper-bullet-specs#.UFiouK4sCSo
Anyway, as others have said the .455 "Manstopper" round was simply a big lead hollowpoint bullet, cupped at both ends so it had some small stability in flight. The 218-255 gr., or so, round went about 600-700 fps (in that range). The British believed in big and quite slow.
At the time the British developed it they were going through a period where they were obsessed with stopping power (or so some authors state) and the big hollowpoint was a part of that trend.
The bullet did not earn the title
Manstopper it was given it by it's makers, Ely I believe. The round never saw service that I've heard of. Introduced in 1898 or so it was removed in 1900 or so as violating the Hague Convention. (Or to look at it as it was, the colonialists and imperialists signed a treaty between them outlawing ammo a couple of years after smokeless powder rendered big bore lead bullets at moderate velocity of any type useless as military rounds.)
The "Manstopper" was tried in the Thompson LeGrande tests but passed on as not as effective as the 45 Colt.
It's kind of interesting that when the British moved from the 455 loads they eventually went to the 380/200 which was a 200 gr. bullet in the 38 S&W cartridge (not the 38 Spl.!) which went about 650 fps or so. They claimed it worked as well as the "Manstopper", at least in shocking power.
Here an old thread on this...
http://castboolits.gunloads.com/showthread.php?t=80187
tipoc