John:

I've kicked this around a lot. Why? Who knows.

Make mine a smoothbore with choke tubes...rifled also.

It already exists and there are 2 options.

Take your pick:

.550 bore:

https://www.brennekeusa.com/hunting-ammunition/brenneke-28tm/

.615 bore:

https://www.brennekeusa.com/hunting-ammunition/magnumtm/