Mac:
Sleeving the .22 barrels into the monobloc requires fairly difficult firing-pin work, however sleeving the .22 tubes into a pair of short off-set intermediate sleeves, and then sleeving them into the monobloc (striking up a neat taper), has two big advantages:
1. Original firing pins can be used,
2. The distance apart at the muzzles is reduced (if only a little), hence reducing the width of the rib (which will be looking bloody wide and flat in a .22 since almost no convergence is required, and the barrels can be slimmed down enormously at the muzzles for weight and handling considerations).

Great project lightningtwice, we will be expecting photos in the near future!!!