I'm going to go out on a limb, here. I've looked at the pictures on this forum, and at the"other" forum. From what I can see without examining the gun, it appears to be a shotgun that someone has rifled later on in it's life. But I could be wrong, having not seen it in person.
While this may not cause any problems, and it may shoot just fine, I would stay on the light side with loads for a start.