DriftwoodJohnson - Great insights into the 44-40. Do you find that fouling is worse with a blackpowder loaded 45 LC?
I don't own a rifle chambered for 45 Colt, so I have no direct experience with that.
A rifle is basically a pipe. Seal the chamber well and all the fouling goes out the muzzle or stays in the bore. If the chamber is not sealed well, some of the fouling will 'blow by' as has already been stated and get into the mechanism.
As I stated, the thinner brass at the neck of the 44-40, and 38-40 for that matter too, allow the case to expand very well to keep all the fouling out of the mechanism. The brass at the mouth of a 44-40 tends to run around .007 thick. The brass at the mouth of a 45 Colt tends to run around .012 thick. I have measured lots of them. So given that the pressure produced by a 44-40 loaded with Black Powder will be very similar to a 45 Colt loaded with Black Powder, the thinner brass will tend to seal the chamber better and keep the fouling out of the mechanism of a rifle. Even though I do not own a rifle chambered for 45 Colt, I shoot with guys who do all the time, and they often have to clean the insides of their rifles more often than I do. My 44-40 brass ejects without hardly any fouling getting into the rifle mechanism.
A revolver is not a pipe, there is a gap between the cylinder and the barrel, so no matter what cartridge you fire in it, soot is going to get everywhere. Until recently, all my CAS revolvers were 45 Colt. And yes, fouling got everywhere because of the open nature of the mechanism. A few years ago I lucked into an antique Merwin Hulbert revolver chambered for 44-40, my first revolver chambered for that cartridge. Despite the fact the the 44-40 brass seals the chambers better in the MH, so much soot comes out of the barrel/cylinder gap that it is a moot point. I can't say a revolver chambered for 45 Colt fouls out any worse than a revolver chambered for 44-40.
But it makes a difference in a rifle.