For a lot of reasons, a carrtridge cannot always be used to its full potential. A cartridge like the .45 Colt is loaded at the factories with smokeless powder, but increasing velocity and pressure would blow up the old guns, so pressures are kept to BP levels. The .45 ACP could also be loaded hotter (and has been), but only at the risk of damage to the guns since auto pistols are made to operate in a fairly narrow pressure/velocity range.
Increased velocity was only one of the reasons to go to smokeless powder, and the change would have been made even if no velocity increase had been achieved. The change to smokeless powder made possible the small bore rifle, not only because of increased velocity, but because it eliminated the fouling that would have rendered a black powder .30 caliber useless in short order. (The .303 British was first loaded with black powder, and the "Tommies" were not happy campers.)
The main difference in the pressure curve of black and smokeless powders is not only in the actual peak pressure, but also in the fact that smokeless powder burns slower and keeps the pressure on further down the barrel. (This is a good point to remember when dealing with old Damascus barrel shotguns. When fired with smokeless, they often let go where the barrel becomes thin, since the barrels were not made to hold much pressure that far out. That that point is where the shooter has his fingers is the reason most folks advise against shooting those old guns.)
As to cleaning, yes. Black powder cartridge guns have to be cleaned pretty much the same way as muzzle loaders, but they are usually more complex and harder to clean properly. A good reason not to load modern guns with black powder is that they are not made to be cleaned the way black powder guns should be. As an experiment, I once loaded some .38 Special with black powder and fired it in an S&W Model 28. What a mess! With the S&W I couldn't just hose it down, so cleaning was a real lesson to not do that again.
"Back in the day", they used a lot of cleaning solutions, most of which were really water based. They didn't slop water around for the reasons mentioned above, but there was much scrubbing with brushes and patches, then oiling. Corrosive primers were the only kind, but were not as much of a problem with black powder because there was a lot of fouling and most cleaners that took out the fouling also took out the corrosive salt (water based, remember, and water dissolves the salt).
When smokeless powder came along, people didn't clean as well and barrels started to rust, leading most folks to believe that the new smokeless powder was causing the rust. A lot of old timers wouldn't use ammunition with that "new fangled" powder because it rusted the barrel. It wasn't until the 1920's that it was realized that the corrosion came from the primer compound, not the powder.
FWIW, I consider loading fixed cartridges with BP to be ridiculous. There are no cartridges for which safe smokeless loads have not been developed. In fact, I don't use much BP anymore at all unless I just want authenticity. There are much cleaner substitutes for shooting. (I do use BP in a rifle musket - it just wouldn't be right without that cloud of smoke.)
Jim
