You could go with either 303 British or 7.62x54R. The 303 would be the more classic way to go. The 7.62x54R would work fine too and allow you to share ammo with your Russian bolt actions, not to mention these days it is easier to find in the US. I'd go with the 303, but that would be up to you and they both have about the same ballistics. A strong 20ga would likely work as a donar. But before starting, I highly suggest Ellis Brown's book.