Thanks Curl.

JPK:
several years ago I contacted Verney Carron about making a fully-rifled 12-bore. At the time, they were offering 12-bore guns with one barrel rifled, the other smooth, erroneously named 'Paradox', only I wanted one with both barrels rifled. I also indicated that I would be happy to take delivery of the gun without ribs fitted, so I could regulate it myself. Perhaps this would have breached their rules of proof, in any case they played dumb. I then requested a pair of the rifled tubes as 'spare parts', already ferrulled ready for fitting to a monobloc (essentially grabbed straight off their production line). Again, they played dumb. Wanted me to send engineering drawings, for heaven's sake! In the end it was all too hard. They no longer offer the fully rifled barrels to my knowledge.

BB:
I have fitted a sight like the one in my crude drawing to a Lee Enfield, and I can assure you that it does work as a proper ghost ring. When counter-bored, you can only 'see' the very front face anyway, exactly the same dimensions as a 'wedding-ring' type. Perhaps I should photograph the 'Smelly' sight on the weekend.

Thanks all for your considered responses.