Ken,
Welcome to GBO. Yes you can drill and tap a 20 ga barrel no problem. It is the same dimension over the chamber as the 20 ga Ultra Slug Hunter. Just make sure to take it to a good smith or only go as deep as the factory does. I don't have one without a scope base on it right now or I would tell you how deep that is. Maybe someone else has one they can measure the depth for you. Do yourself a favor and get the H&R/NEF base, it has more slots and options than any of the after market ones do.
