That is a great looking mold Ed. I don't know if the .620 diameter is the best option though as I would rather be able to size a slug down than hope that it bumps up to diameter.
Looks like a topic for another thread.
Huvius mentioned a single barrel trap gun. I believe that the perfect hammerless stalking rifle type gun can be built on Savage 219 or 220 (old model)
If you look inside it isn't much different than the small frame snap action German tell rifles.
They were chambered to cartridges as large as 30-40 Krag so strength is no issue.
Barrels can usually be refitted from Savage 94 single shots if you are monoblocking. It also gives an opportunity to have a gun with multiple barrels including rifles.

Cheers.

Tom