Mike A.,
My cousin and I never could get good results with the hollow base bullets in his .380 Martini Rook rifle.We didn't try Black powder,however, which may have made it work.I had a mold for a conical bullet intended for 36 cal cap and ball revolver.The base was rebated enough to use it as a heeled bullet(lubed with Alox).It was satisfactory, but had to be handled carefully.I think Buffalo Arms may have 38 Colt heeled bullets, maybe even a mold.I still think this should be seriously tried first.Re-regulating this double to .22 would be a "trick", even with the little convergence for low recoiling 380 Rook.Again, just my opinion, it's not my rifle.
Mike