I recently bought an 1873 Maynard that has a shotgun barrel. The muzzle measures .61". I made a dummy round out of a plastic 20 ga shell (no powder or shot) and it slid in easily, and fired the primer on the first try.
So, are the Magtech 20 ga brass shells the same size as the plastic shells? I can't find any online but I'm sure they'll be available at some point.