I can only speak to this issue from experience with purpose built double rifles.

While many "best" shotguns have no rib extension, I believe that double rifles should and I try to avoid those that don't. If it has a third fastener of the screw-grip type, great. If not, I prefer that it have none at all as, from observation over the years, I don't think any of the others work.

A friend of mine has a British .450 No. 2 NE that has a dolls head, but no third fastener. It was 90+ years old when he got it, and he's put 3000 rounds through it since then, and it's still tight on face. Likewise one of mine has only a dolls head with no third fastener, was old when I got it, and I've put quite a few rounds through it since then, and it's still tight on face. I would guess that I've seen as many DRs with third fasteners off face as I have without, except for the screw-grips.

Perhaps since the converters are using shotgun actions, they think a third fastener will make up the difference. I think they're kidding themselves. Either the action is strong enough or it isn't. At the range, when folks shoot shotguns converted to DRs, I go hide behing a nice cast-iron engine block.

The need, or not, for a third fastener has nothing to do with bore size.
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