The big bore Webley: A fun revolver.
I've had three Webleys - -A MK-IV in .45 ACP, and a MK-IV and a MK-VI in .455. Wish I had at least one of them now.
The .455 Webley round was not so anemic as one might think. It stacked up quite well against most military cartridges of it's era of origin. The original .45 Colt SAA was pretty stout, but almost all the military versions in the Indian War period were used with the .45 S&W Scofield round, which was not nearly so powerful. The .38 US Service revolver round was nowhere nearly so powerful.
As the old Webley revolvers were made of softer steel than modern arms, they will not stand up as well to heavy loads, certainly.
The .45 Auto Rim cartridge usually gives somewhat better accuracy in a converted Webley than factory .45 ACP. The Webley bullet was a true .455 diameter, and the hard jacketed ACP ball, at .452 diameter is distinctly undersize, and being hard, will not "slug out" and take the rifling very well. The factory lead Auto Rim bullet at least obturates somewhat. The best solution for the shooter of a converted Webley is to use Auto Rim cases and, if you can find them, the Winchester 255 gr .45 Colt factory bullets. They are a hollow base bullet made of soft lead and 5.0 gr. of Unique pushes them at slightly over 600 fps. Also, the heavier weight is closer to original specs, and the point of impact is closer to point of aim with unaltered sights.
Another possibility is to find someone with an old Lyman 454424 mold and either shoot lubed but unsized, or order a .455 sizing die. If you are lucky enough to locate some boxer-primed CIL .455 cases, back down a bit on the powder, as the .455 case is only .757 long, compared to .886 of the .45 ACP and .897 of the Auto Rim.
The above applies to the break top Webley revolvers. Those with .455 S&W or Colt New Service revolvers can load hotter if they wish, but it is not recommended. If one wants to shoot at .45 Colt or magnum power levels, those revolvers are more common than the old .455s. There is a lot of fascination in "doing it the way the old timers did it."
On the other hand - - I believe a Webley MK-IV made of modern steel would be very combat- worthy. It could be .45 Colt or .44 Special, but I'd really like it in .45 ACP/Auto Rim. The stirrup latch can be operated with shooting hand thumb and the barrel broken downward against the leg, ejecting the empties, while the off hand takes a moon clip from a speedloader case. One quick movement inserts the loads, the barrel is slapped upward, and you're ready to rock! All the Webleys I've ever handled had very smooth actions, though some had rather hard trigger pulls.
Best of luck in shooting the .455.
Johnny