The Merkel should do just fine. Merkel only uses one frame for their double rifles and shotguns. So in essence they build their shotguns on a rifle frame. I had made myself a nice 9.3x74r on a Merkel Model 8 20 gauge. I however let myself get "talked" out of it at the local range one day.

The "New Rigby" or "California Rigby" uses all Merkel actions to build their lines of rifles. http://www.johnrigbyandco.com/pages/SidelockD.html They chamber eveything up to 577 NE in their line.

Bill