Re: Webley Mark I
The problem with firing those "shaved" revolvers with .455 ammo is not with the pressure, but with the unsupported rear end of the case resulting from excess headspace. If the revolver is loaded with .455 ammunition, the cartridge will fall into the chamber until it is stopped by the rim. But when the round is fired (assuming the firing pin can reach the primer), the case will move back, and part of the case will be unsupported, possibly resulting in the case failing and releasing gas. Even at the low pressure of the .455, that would not be good and could result in injury to the shooter. That will not happen with the .45 ACP case, which has a stronger head than the typical British .455, but the .45 ACP standard load runs over twice the pressure of the .455, inviting trouble in the old revolvers.
I reiterate my advice; fire those guns only with lightly loaded .45 ACP and clips, or with lightly loaded .45 Auto Rim.