[Rupert]

Note that one of the major problems with fouling was in loading - it made it increasingly hard to force the ball/bullet down the bore of a musket. However, when you're using a cartridge-loaded breechloader this is no longer a consideration.

Thus for a breechloader the consideration is the point where the bore is so fouled that pressures rise to a dangerous level or fouling of the mechanism jams it. The former doesn't seem to have been a consideration, even in the Lee-Metford, which had a .303" bore (quite small for the time). The latter shouldn't be a problem, as none of these weapons were opening the breech before chamber pressures had dropped to atmospheric.

I don't think there's a real concern with a metallic cartridge weapon or a revolver as long as it's cleaned promptly and thoroughly after the action is over. The malfs listed in HT and other sources for particular weapons cover any issues sufficiently.