What a beautiful early rifle.

Yes - 11 bore is .751" exactly.
In the States, Parker Bros. made 11 bore shotguns for a while.

As Tinker suggested, the chambers, lengths, and sizes are important. You might be able to use 12 bore brass - who knows? They are probably for brass cases, unless there paper cases were made that early? Weren't some 'boxer'(Snider) cases made of thick paper with steel bases around that time?

It will most likely be meant for ball but capable of shooting both, but you will want to check the twist rate. At that time, most guns were sent out with both round ball and a minnie-type bullet moulds some with a press for properly sizing the bullets.

Some British Gun Makers had some wierd ideas on twist rates back then. The invention of breechloading double rifle capable of shooting fixed cartridges made use of elongated bullets of harder alloys possible and safe to shoot and thus made bullets capable of being used on dangerous game.