I was just wondering if a webley mark 1 converted to .45acp safe to shoot the modern ammunition or is the conversion just done because of the ready supply of .45acp brass? I don't know a whole lot about antique fire arms and am just trying to figure out what would be a good buy.
The .455 Webley cartridge has a maximum pressure rating of about 12,000 psi, while the .45acp has a rating of 18,000 psi. The old Webley MKI and MKII service pistols are designed to handle the webley .455. The pressure from a full power .45acp is 50% over what the pistols were designed for.
On the other hand, many MKI and MKII webleys have fired a lot full power .45acp and haven't blown up yet. It's a robust design and chances are it won't blow up in your face, at least not right away.
It's easy to spot a MKI that have been fed a steady diet of fullpower .45acp because they are loose in the frame hinge and latch. Those 120 year old frames are made from softer steel compared to modern firearms and over-pressure loads stretch the steel in the stress points of the hinge and latch assemblies. Also jacketed bullets will wear out rifling in these soft barrels much faster than soft cast lead bullets for the same reason.
The Webley MKI and MKII .455 / .45acp revolvers are like pressure vessels. If you run them at 150% of design pressure they fail quicker and wear out faster and there is always the possibility of a catastrophic failure.
SO - my answer to your question is: chances are good that full power .45acp will not blow your face off, but it definitely will cause cumulative damage over time and it will wear out your old pistol much faster.
Somewhere around 5.0 gr of Alliant Unique powder with a 250 or 265 gr .455" bullet in a .45acp case will give you fairly close to 12,000 psi, and still give you a very respectable load without damaging the old pistol.