Don't rely on the forward edge of a 455 chamber to reliably headspace any round. It was never intended to. It isn't cut to a square edge as in a 45acp or 9mmL.
Chamber specs are very different from it's cartridge specs and reading the latter and expecting the former can lead to trouble.

The Webley conversions in particular can be very nicely done,,or can be almost brutal to look at.
These revolvers were done by the thousands as cheaply and quickly as possible just so they could market them in the USA to sell for $12 out of a magazine ad.
Small individual workshops were the primary source of the conversion work.

Some are way out of spec w/ excessive headspace w/the 45acp/AR.
It pays to do a bit of measuring before shooting these.
Headspace in a revolver is just as important from a safety standpoint as any other firearm


IMHO, avoid commercial 45acp ammo in the conversions.
Webley cylinders of any Mark are like finding a diamond in the desert now. Damaging a cylinder on one pretty much leaves you with a parts gun. Once in a while you can find a cylinder though but you pay dearly.
I have the remains of a couple I've replaced for people over the years, the rest were gone. All from commercial 45acp ammo.

I wouldn't shoot the stuff in a S&W 455 converted either, but that's just old conservative me. I don't think the 455 had the same heat treat as the 1917,,but I could be wrong.

Take it easy on 'em,,their around 100yrs old or more and most have been through a couple of wars,,at least.