I have no experience with black powder weapons, so take what I say with a grain of salt. Generally speaking you will want a longer barrel. 6 inches is more ideal. The reason for this is that the powder, especially black powder, burns much slower than you would think. As a result short barrels like 3 inches and below don't give the bullet enough contact time with the pressure front caused by the detonation to reach full potential. With 3 inches a lot of the powder will not be burned yet by the time the bullet leaves the gun and the explosion can no longer impart any more power to the round. 6 or 7 inches is going to give a lot more speed to the bullet which is going to provide a lot more power when it hits its target. As for round types. I think cylinder is probably better. Ball may have lower aerodynamic cross section which may make it fly a bit further, but at close range it makes little difference. Choose whichever is heavier. As for material, lead 100% over steel. Lead will make a better seal with the barrel as it is softer, letting more of the energy affect it. Steel also does not deform much on impact, especially with a soft target. Steel is ideal if you have to punch through armor or hard objects, but on soft tissue lead is the best. You want the round to hit the tissue and effectively "splatter" and stop moving. If the bullet stops inside the target, it means that 100% of its kinetic energey has been dumped into that target. Driving a nail into the bullet before hand will have no discernible effect. Most of the damage to soft tissue is going to come from hydrostatic shock caused by the fast moving object moving through a fluid with causes a shock wave of sorts inside the target. Sorry if this is too wordy and too much info.