Please consider these facts -
1. The operating pressure for the Mark VI Webley revolver (the last, and strongest, of the .455 Webley service revolvers) was a maximum of 13200PSI (i.e. six 'long tons' of 2200 lbs).
2. The standard operating pressure generated by milspec and full factory loads of .45 ACP ball ammunition is 19,000PSI.
3. The pressure of .45 ACP milspec and standard factory loads exceeds the proof load for the Mark VI Webley revolver.
4. The dimensions of the chamber throats on Webley service revolvers are a bit variable, but generally are smaller than bore diameter for some reason - but this system worked well with the original hollowbase bullet design, which was quite soft (20/1 lead/tin) and non-jacketed. These soft bullets swage down passing through the chamber throat (.... my own view is that this was intentional, to get the most out of the relatively small powder charge, and thus maximize the velocity produced ....) but then the base expands nicely to engage the rifling in the bore. However, the general consensus is that jacketed bullets (such as those loaded in most GI-spec .45ACP ammo) can work to dramatically increase the already excessive chamber pressure generated by such rounds.
5. Furthermore, the earlier models of .455 Webley service revolver were not as strong as the Mark VI. Indeed, the Mark I, Mark II and Mark III revolvers were in fact designed for black powder loads.
