Absolutely marvelous, Mike!

I'd be experimenting with round balls - I would about lay odds your rifle would regulate RB's at Cape Buffalo stomping speeds - say 1,600fps to 1,650fps. The recoil would be just slightly higher than the Brenneke slugs, which I think might be necessary to get the barrels to shoot together.
I would use a .710" or .690" ball, held centred by a plastic gas check with cup under the ball. That way, no leading, and yet the ball will be centered and spun by the rifling.

Just a guess on the regulation, mind you.

Trackofthewolf.com sells round balls in may sizes, 25 to a bag = perfect for testing.