If we take this discussion for what it's worth, I come away with a few points:

-- of all the failures mentioned, every one was due to a pierced primer via a deformed firing pin; the issue was equally spread between conversions, shotguns, and double rifles. This caution belongs then in the same category as bore obstructions and light triggers.

-- youth is no barrier to skill; if one Youtubes the various makers, Krieghoff through Holland and Holland, half or more of the makers appear under 45 and many under 40. Condescension leveled on youth is simply unwarranted.

-- double rifles on shotgun actions have a long history; Merkel and Chapuis at least using the same systems for their rifles as their shotguns...CptCurl's Merkel looks to be a 500 Nitro on a 12 gauge frame, according to the Merkel website. I could be wrong on this, and it also bears noting that they build shotguns on their rifle actions, not the reverse.

-- double rifles are not expensive due to manufacturing, think Baikal double at roughly $700 CAD. Rather, they are expensive due to labor in regulation, fit, and finish. None of these are mechanical concerns. It's not a black art.

-- proofing at least at Chapuis is a shoot loose exercise; shooting loose appears to be an accepted method.

-- people are less concerned with getting stomped by an elephant than they are being sued. This puzzles me. I say, sue the elephant. Extract some dime from his sorry grey ass.

-- nobody is above being questioned, ever; and

-- if it concerns you, don't do it.

I had my doubts on whether it was safe to build a double rifle on a shotgun action prior to this thread along the lines of one of the more vigorous commentators. Further research of the topic seems to indicate that it's as safe as these things go provided modern materials are used and due caution observed; it is a process done by many companies and many smiths successfully and often. Certainly it is not an unreasonable mental consideration.

I feel it's also crucial to compare apples to apples; JV Howe for instance looked very much down his nose at classic American shotguns, comparing them to classic Belgian shotguns. To apply a one size all approach is like saying an old rolling block is the same as a Hagn, and that's just not correct. Comparing a modern double to an ancient mutt is the same.