I have been wondering how it is going also since reading this thread today.
I have been wondering about building a double rifle from a doner 12 ga db for some time. If it could be worked out? A reamer for the 12 ga bbl, tunn down the rifle barrels and get a set of threading dies built for the threading inside and outside threads,(male and female) for the mateing, and a regulator end clamp for POI with a front sight on it. It could accept barrel blanks from any acceptable caliber pressure wise. Something for a real gunsmith or dedicated hobbiest. I'm way short of trying it at this time though.
I'm interested in 357 max or 454 casul.
I hadn't heard Remington might offer doubles, inexpensively. That is real interesting also.
I like double barrels. eddiegjr