The American .45 ACP was the contemporary of the British .455 cartridge.

One BIG difference, however, is that .445 British started out as a black powder cartridge more than 20 years befor the .45 ACP was developed. Th .45 ACP was started as a smokeless powder cartridge from the beginning. Hence, the .455 British round has the benefit of being developed for use in black powder designs, and evolving into smokeless powder revolvers (Webley MkVI).

Although almost identical in outward appearances and with similar ballistic performance, the chamber pressures are considerably different.

The .45 ACP operating pressures are near what the .455 Britsh proof pressures are. This makes shooting .45 ACP ball ammo in converted .455 revolvers an "iffy" proposition. The S&W cylinders of this period were unheattreated steel.

The contemporary of the M1917 S&W is the British Webley or Enfield (Webley design built at Enfield arsenal) MkVI revolver. Of the many tens of thousands released as surplus after WII, many were converted to fire .45 ACP in half moon clips. Of these conversions, many have developed burst or bulged cylinders from firing the higher pressure rounds.

I have on good authority, heard from Canadian collectors and shooters of the converted .455 that split cylinders are fairly common, as stocks of factory and surplus .455 ammo dried up and shooters had their guns converted to fire the common .45 ACP.

The more prudent shooters (including myself) choose to reload mild 200 grain cast lead bullets to 750-800fps in .45 ACP for use inverted MkVI and M1917 revolvers. You will get no argument from me that the 93-95 year-old revolvers were made when steels and heattreatments were less strong than they are now.

Please be advised that if you go with the .45 ACP conversion, it is not wise to fire .45 ACP ball or high performance handloads. They will certainly chamber and fire, but you will essentially be firing .455 British proof loads with each shot.