When your talking large bore, are you refering to the .54 and .58 calibers? There are many rifles out there that will shoot those size balls. Green Mountain Barrel Company sells drop in barrels for a T/C Hawkins/Renegade that can take you from there all the way into the smoothbore end of the game with the .62 caliber and will make anything you like.

Your other option will be to go after the older Revolutionary War era muskets, the Brown Bess in .75 I believe it was or perhaps .72 caliber. There are also rifles on Dixie Gun Works that get into the larger calibers, the problem with most of them are, they are muskets not rifles.

As you are aware, muskets although accurate to short ranges are not known for there long range accuracy. They do have some tremendous roundball power, but if memory serves me, a standard load for most of the large bore roundball long guns was 70 grains wad and ball.

For instance, I have two .54 caliber rifles. They are considered by some as large bore rifles. I shoot roundball out of them. I load 85 grains of T-7 in one, and with a roundball this thing really has some knock down power.

Hope this helped