Black powder burning under high pressure with a tight patch ball combo, deposits less fouling than a loose patch ball combination. When a significant amount of powder is still burning when the ball exits the muzzle, fouling also increases.
efficiency can lead to less fouling, but there is still fouling.
I once took part in a stake match and for speed in loading, just loaded bare round balls. The fouling build up was horrible and after 5 or 6 shots, I had trouble loading a ball, even with a stout ram rod. and it took me an hour to scrub the caked up fouling out of the bore. Powder burning at lower pressure from a loose combination, leaves more fouling. I think part of the fouling in your case could be a loose combination.
In barrels between 40 and 50 caliber, adding ten grains of powder increases the resulting velocity until the charge reaches a certain point. At that point the velocity still increases, but at a lesser rate. For instance, in a 50 caliber 26 inch bore, each ten grains from 40 to 70 grains may produce an extra 120 ft / sec per ten grains, and suddenly about 90 grains, the increase in velocity drops to about 80 ft / sec and with the next ten grains, the velocity drops to 60 ft/ sec additional. The point where the velocity increases but not as much, is sort of what some call a "point of diminishing returns". For most round ball guns, that point is about the most accurate bench load. But not all. Nor is it the most powerful hunting load. Depends of rifling and twist.
Once calibers drop below 40 or go over 50, the points of diminishing returns start to blur. In a 54 with some powders, there is a point in a 32 inch barrel of about 110 to 120 grains. But that is more than most shooters will load anyway, even for hunting. With a 54 gun and some powders, the velocity increase keeps climbing to the point that the load is dangerous without reaching that point of diminishing returns.
But there is another "efficiency" point that some forget about. When velocity exceeds the sound barrier, more air builds up in front of the ball and causes it to lose velocity faster as it travels down range. Now the super sonic ball is launched faster and remains faster down range, but by the time it passes the hundred yard line, the faster round ball has slowed significantly more due to air resistance. You may double the powder and get almost twice the muzzle velocity, but at 100 yds gain only a 20 percent increase in velocity. Flatter shooting yes most definitely, but the range of ft pounds energy may only be extended another 20 to 25 yds over a much lesser load.
