Re: barrel length
Black powder burns more slowly than smokeless. The other variable not present with smokeless guns, is that muzzleloaders can be loaded with widely varying charges.
In a 50 caliber 28 inch barrel, you reach the point that not all the powder is consumed before the ball exits the muzzle around 85 to 90 grains. Pressures and velocities continue to increase with each additional 10 grains, but at lesser increases than the ten grain increments under that 90 grains point. So if you dump 120 grains in a 28 inch barrel, you never get 50% increase in velocity over an 80 grain load.
Longer barrels give additional bore length to consume powder permitting higher velocities with the same powder charge and add sight radius to help with aiming.
I see frequent posts about guys loading 130 grains in their short barreled guns trying to extend range or flatten trajectory. The fact is that such efforts do not add significant range or flatten trajectory that much. a portion of the additional powder is just burned up in muzzle flash.
I shoot a long rifle with a tapered barrel. It is 5ft over all, but weighs less than the average TC Hawken. while the extra length may be a hassle walking through the brush, it is lighter. If I were the type to load such heavy loads f powder, at least my gun would have the barrel length to get the extra oomph out of the powder. If you stick to 90 grains or less, the barrel length over 26 inches really doesn't matter much.