OK. Allow me to repeat: "The idea is to have THE single most versatile firearm in the world".
You're way underthinking how to reach my goal. But 16 ga>12 ga might be a reasonable option.
A rimfire adapter in a shotgun barrel is useless. Will not be accurate. At all.
Actually, I agree with Erich. I don't think your configuration is optimal. As well as the issues erich outlined I don't think that your .22 Wyld part twist, part straight rifled barrel is practical, and you seem to be providing for carrying rather a lot of gear and different ammunition to support this setup.
Rimfire adapters for shotguns aren't confined to the MCA type. The Germans and others have for many years been making adapters for shooting rifle calibres accurately from shotgun barrels. They can also be adjusted (unlike your simple MCA-type) to ensure that the insert barrel shoots to the sights. Google "einstecklauf" - that is what they are called, and they are available in shorter (8" - 10") and full length versions - the former are much handier.
You could still use an MCA-type adapter to shoot .22 shot in a shotgun barrel though, and not need to have your rather overcomplicated .22 Wyld solution.
Personally I would choose a drilling in 16/16 over something rimmed. My own drilling is 16/16/7x57R, but you could also have 7x65R or something else if you wanted it. Yes, 16 ga ammunition is rather less widely-available than 12, but 16 ga drillings tend to be slimmer and can be lighter - mine's 6 lb bare and handles like a good double shotgun - and a 16 ga does the job for me on anything from pigeons to pigs. In any case the very concept of "survival" presupposes that you will have a stock of ammo and won't be in a position to be going to the shops to buy it.
A rimmed case for the rifle makes for a simpler extractor and a case which drops free from the chamber when you are in a hurry. I like the 7x57R, and have shot everything from rabbits - headshots neatly decapitate them, without destroying the edible part - to pigs and deer with it. If the SHTF I could also drop larger game or stray livestock with it too.
If I was inclined to try shooting "tweety birds", whatever they are, I could get a chamber adapter and use "ratshot" loads. If I wanted to use .22LR to pot small game I could insert an einstecklauf in the right barrel, adjusted to line up with the sights. In practice though I just keep it simple: some rounds of 7x57R and some 16 ga in a couple of different loads - usually some size 6 for birds and rabbits and some buckshot for pigs. I could easily pocket a couple of slugs too. For roughly the same amount of work as it would take to rustle up a feed of small birds I could drop a deer, goat or pig and have meat for the week, if I could keep it, and if I was worried about being discreet I would, as others have suggested, use a snare or trap or catch a fish.
