If you think about it, a percussion revolver is a breech loader. Muzzle loading rifles use bullets/balls that are bore diameter or less (if patched) so they can be loaded into the barrel. Breech loading cartridge guns use bullets that are groove diameter so they have to be engraved by the rifling, giving the best accuracy and seal.
But a percussion revolver is not loaded from the muzzle; its chamber effectively acts like a cartridge case. And the ID of the chambers should be the groove (not the bore) diameter of the barrel. Bullets/balls must, of course, be sized accordingly. When a smaller bullet/ball is used, it might or might not expand (upset) enough to fill out the grooves.
FWIW, the chamber mouths of a nearly-new condition original Colt 1860 measure .4505". Civil War conicals and balls generally ran .451-460".
Jim