I guess I should have said the 'Gun's' action, not the 'rifle's' action. I meant gun.

Since you will be using normal wads, a jug-choke of about .020" will improve patterns and I found in a .44 smoothbore with .005" jug, it didn't hurt RB shooting in the least. Others with jug chokes in muzzleloading 20 bores get good results with round balls and shot.

Should also be easy to make nylon or HDPE sabots for a 1" bore. Slit lengthwise like a BRI sabot, they would expand or split out upon firing to touch the bore itself for straight guidance & release well at the muzzle with a mild cone cut in the nose, like the BRI's.

About any large projecile could be used, I guess - how about 3, .735", 600gr. round balls for a buck-shot load?

That ought to leave a mark on something at 1,500fps to 2,000fps.