Friend has a revolver in .455 Webley that has had the rear face of the cylinder modified to enable him to shoot .45 ACP cartridges in it using half-moon clips. He has asked me to load some rounds for him using light powder charges and cast bullets. I have some concerns--should I have concerns?

Read on another site that the .455 uses .476 diameter bullets, so that would seem to mean the .45 bullets would "rattle down the barrel" when fired.

I'd have no issues with it.

Just make sure that he supplies the brass. Have him buy a box of new 45 ACP bullets and shoot them. Then give you the empties. If the gun can digest a box of factory rounds I'd have no qualms about shooting a bunch of light loads in it.

If he doesn't want to do that, all I'd load is cast bullets over a light load of black powder.

The Webley uses a .455 hollow base bullet. The 476 was one of the sizes used during the inside/outside lube, black/smokless powder conversion days. The throats are small so the hollow base is needed for accuracy. The standard max load with 255 gr bullets is 6.5 gr Unique for 680 fps, or 4.5 gr P38 for 690 fps. These were taken with my MKVI with a 6 in barrel.
Good Luck!

Mark & Hawkins--thx for the input. With a .455 bore diameter, wouldn't I be better off with .454 dia bullets? Or just accept the lesser accuracy potential with the .452/.451 projectiles? And, not sure if I should even worry about accuracy at this point as the gun will be strictly a "plinker".

I mis-typed, the Webley uses a 265 gr hollow base bullet. Didn't have my second cup of coffee. The powders and weights are correct.
Hornaday sells a 255 flat base that is sized to 454. Also grafs sell a 185 hollow base heavy plated .452. I have some but haven't tried them yet. Goog Luck!