When I was a kid I learned to shoot my H&R single shot 28 gauge holding an extra shell between the fingers of my forend (left) hand. Now with my side by sides I don't bother.

When I am at the range with my Ruger No.1 in .450/400 NE I revert to the old practice, for two quick shots in sucession. Haven't hunted with the gun yet but would do the same in the field so as to have the option of a quick follow-up. I honestly feel that I can reload almost as fast and reliably as I could cycle a bolt action. But again, when I shoot my double rifle in .470 NE, I don't bother.

I believe they also now make cartridge/shell holders that you can wear on your forearm. That way with the big bores you can concentrate on gripping and hanging on...

I guess I feel if you can't get it done with two shots, then the game gets away. In the case of DG that fight back, it is probably helpful if the PH backing you up has at least two shots as well.