Webley rounds aren't particularly powerful, either in .380 or .455. Their operating pressure, about 13,000 psi, is lower than .45 Auto and on par with a lot of black power cartridges.
They may look big and beefy and strong, but more than one Mk VI converted to .45 ACP post World War II has left loose.
It's particularly frightening to see earlier Webleys converted to .45 ACP. Anything from a Webley Mk IV and earlier is a disaster waiting to happen if fired with anything other than blackpowder cartridges.
When the British adopted Cordite, Mk IVs and prior Webleys had to be withdrawn from service and replaced with Mk Vs, as even low pressure Cordite was too much for them.
