Are the Merkels built on 20 gauge shotgun actions or are the 20 gauge shotguns built on Double Rifle actions?

All classes of cartridges should be built on an action appropriate to their size. One size does not fit all if you want a nicely balanced rifle. A 375 built on the same size action as a 470 results in either a big clubby 375 or a 470 on too light a frame.