I used to load a .720" patched roundball over a case full of BP in my NEF rifled 12 ga, and that worked reasonably well. Basically a muzzleloader you could load from the rear. It would melt the plastic cases a bit, but they're free on the ground at the skeet range so I didn't care. I didn't crimp them, just seated the ball with firm pressure just like I would in a muzzleloader. It would hold 4 inches at 100 yards, good enough to whack Bambie with!