Seyfried had an excellent article on a 12 bore H&H Hammer Parodox in Handloader or Rifle magazine some time ago. Just re-read it a while back. Remember it?

He was at a loss to get it to shoot well until he came across a sizer which made the bullets .001" larger than the bore size. From there, it was merely testing with different wad collums around 1,000fps that got it shooting properly.

I'd really like to test a Parodox with round balls. I think they'd increase the power considerably, as heavier loads than slugs would allow, would be needed to get the barrels to shoot together with the balls. Heavier loads means higher velocities, flatter trajectories and more killing/stopping power. One could easily load patched balls in such a gun's cases to adjust size according to the bore's dimensions.