Marrakai, congrats on having a go. I've been daydreaming of doing one of these for some time, just never got around to it :-) . I was looking at a Boito (shudder) donor gun. Some of them had the firing pins going through screwed in bushings. That would have enabled replacing the firing pins with offset pins to suit rimfire. I was thinking regulation could be done by trial and error muzzle bushings.

Good luck with the project and keep us all informed on progress.