Joe: A 20 ga. muzzle loading smooth bore with a 2 dram load, and 7/8 oz shot gives a nice easy pattern typical of open cylinder bore guns. If you use a shot collar, you can improve the pattern to about modified. At 30 yds., its killing power is related solely to the size of shot pellet used. #8 shot works fine for dove; #5 works fine for larger birds like grouse,pheasant, and partridge.
As to barrel length, powder charges burn efficiently based on the size of powder used, the amount of resistence provided to increase chamber pressure quickly, and, of course a reasonable barrel length. 24 inches is more than adequate in the smaller gauges with reasonable loads.
Rifles offer a different problem, because the grooves do offer places for gase to escape, and keep the pressure low. They also reduce friction with the PRB, making it easier to move, reducing pressure in the chamber. When that chamber pressure reduces, you get inefficient burning, and more residue, other things being equal. Burning efficiency increases, and Pressures are raised in rifles when a filler is used between the powder charge and the PRB. Part of that is caused by not only the increase in mass, but also the fact that a filler will compress before it allows the gases to move the PRB. That time it takes to compress allows more burning, and pressure to rise in the chamber.
I would always prefer a longer barrel in the larger gauge shotguns, as the reason for using them is to be able to shoot increased shot loads, and that requires more powder, and longer barrels to burn it. A 30 inch barrel works, and a 36 inch barrel on a 12 ga. shotgun, particularly a double is about all you want. If you are shooting a punt gun, which can be 2 ga. or 1 ga. then, you need that 48 to72" barrel, but you also mount the thing on the bow of a boat, and the gun was designed to shoot large quantities of shot over a wide spread so that flocks of duck sitting on a pond, or river could be killed with one shot. Since market hunting is illegal, such guns are also illegal for hunting, and other than curios, there is no reason to have one, or try to shoot it. YOu could do as well with a mountain howitzer mounted on anything, and shooting shot! The 30 inch barrels on the howitzers, which were up to 4 lb. pieces, but typically 3 pounders, could handle as much shot as a punt gun, and be slightly more portable.
