Certainly possible to convert to rimfire, it would also include altering the breech block and firing pin and might require a new block as well. The firing pin is retained by a nut which is threaded into the block on the same axis as the pin. It may be easier to make a new block with the pin in the proper location that to alter the original block. If it was mine I would go back to .360, either 2&1/4" bpe or 2&7/16" bpe. I have a double in the 2&7/16" version which was a favorite chambering of Henry's.
