Wow! I love this discussion. I am an old physics nerd, and I agree the recoil happens as soon as the ball starts to accelerate while still in the barrel - like if you throw a ball while on rollerskates, you start to move back as soon as your arm starts to move forward, not when the ball leaves your hand.
My pistol balls are .595 caliber and therefore pretty heavy, and the barrel is 12" long, which means there is a heavier force pushing back on the pistol for a longer period of time than if the balls were lighter and the barrel shorter.
I was thinking that maybe using a larger powder load (I was using 30 grains) might reduce the muzzle rise by getting the ball out of the barrel in less time, but I suspect the increased force would offset that, and therefore not reduce the muzzle rise.
What amazed me is that PathfinderNC's video () show him hitting a target much farther away.
I'll be going to shoot again in a couple of days, and I'll try to figure out what's going on and will report back.