Here you go. Below link is a place where you might find a wealth of info to fit your needs. I pasted a thread that I think answers your basic question.
They talk about shooting over "snow" to check for unburned powder. As snow is scarce down here just use the bed sheets. Wife wouldn't mind would she?
Long barrels do gain a bit of speed over short barrels and heavier powder charges gain speed over lighter charges from either short or long barrels. Some examples here from the Lyman BP handbook show a .50 caliber with .498" ball and .015" patch with G/O 3f as follows.
50 grains, 26" barrel, 1348 fps
50 grains, 43" barrel, 1506 fps
You see here that both short and long barrels gained velocity right up to the heaviest charge tested, in fact, the short barrel seemed to gain a bit more than the long one from the heaviest charges. Whether or not the gain is worth the pain is a personal decision and I'm certainly not recommending anyone exceed the manufacturer's recommended maximum load. But don't let anyone tell you a 26" barrel can't burn a heavy charge or that there is no benefit to a long barrel even with light charges.
I know that federally, muzzle loading black powder rifles and shotguns aren't considered firearms, so barrel length isn't restricted. I am not sure about the state level though.