Joshua,
The game laws in the US are such that it is the gauge of the shell that the gun accepts can not be larger than 10 gauge.
Since this is a muzzleloading gun it takes no shell.
Remember that there are many shotguns out there that are chambered for the 10 gauge shell that are over bored to 8 gauge, i.e. a chamberless shotgun.
I know of no game wardens who carry chamber gauges, nor do I believe that they pay any attention to those of us who enjoy using such novel pieces to hunt with.

Just my two cents worth.