Brother, from somebody that wanted to go that route and has zero skills (me), there are much better routes to a DR. If you can do the work that might be different BUT its a LOT of work.

My first suggestion is the Baikal double rifles. The O/U if you can still find them were avail in 223, 308, and 30-06. They can or could be found for well under $500. The SxSs are in 30-06 and 45-70. The tend to run about $750+. The Baikals are the AK of doubles, not baaaaad; but certainly not Merkels and the price reflects that. They will also be way less money then trying to build a DR on a shotgun.

Next Id look at the Sabbatis from Cabellas. They have 45-70s and 9.3x74r's for $3000. Thats more pricey but I was really satisfied with my 9.3. Again its not a Merkel or a K-Gun but its a nice gun for the money. The Pedersoli DR is in 45-70 and run a bit more. They are pretty sweet if you dont mind hammer doubles.

I bought barrel inserts for a 12ga. They were another of my do it on the cheap experiments that failed. They had no regulation and no way to extract the cases without poking a rod down the barrel AND the longest available are 18 inches.

Maybe 10 years ago Franchi (I think) made barrel inserts in 30-30 and 45-70 for Baikal SxS shotguns BUT they were only for the Coach Guns, so the barrels were short and they have "nuts" on the front of the muzzle to lock the barrels in place. The worst part was that there was very little regulation and the damn thing is heavy!

Converting a shotgun sounds cool; but Im so glad Im past that idea. Do what you will, and I wish you the best; but I bet youll be happier with either the Baikal doubles of the Sabbati. Theres a Pedersoli in my future....