Andreas-

Barrel-length is going to be one issue there, pump and auto will have short tubes, also the autos will have gas-vents drilled in them at some place.
Also, most everything out there is set up for sub-caliber sabot ammunition - with fast twist.

There are likely properly bored muzzle loader barrels with the right twist that could be contoured and sleeved in as full-length liners, or one could be bored and rifled by one of a handful of US barrel makers.

Cheers
Tinker