Knight makes the TK-2000 12 gauge ML shotgun, which is the first one that comes to mind. It's a current production shotgun. I think that T/C also makes one, and maybe Traditions. I'd check out the major ML makers websites and see what you can find.

I, personally, have seen them, but never shot one, nor do I really have any interest in them. I've read that working up a load that patterns well can be a major pain. And I imagine that they are not easy or quick to load (by ML rifle standards).

Mike