I thought about that - I lined a Swedish rolling block 16ga to 38-55, filling the gap(about 0.020) with liquid epoxy. Single barrel so no need to regulate.
If the 22rf barrels were undersized for some length (10 inches?) at the muzzle end, a set of 3 or 4 long wedges each side could be used to nudge them into regulation, then filling the space with epoxy might work.
Some offset firing pin arrangement should be fairly possible.
Have fun, keep us informed!

Dumb, but learning...Prof Em, BSc(ME), CAE (FYI)