Semi - very interesting concept. Good luck. I do have one thought though. If this rifle is going to have replaceable barrels might I suggest two alignment pins in the receiver that line up to recesses in the barrel so mating of all barrels will be identical and hammer/nipple are in perfect alignment each time a barrel is changed. Or a single horizontal running pin that goes through the receiver and through a notch/hole on the barrel.
Once this sucker is done I will be honored to have a sample for field testing. I would like a .45 with a 1:30 twist and a .50 with a 1:24 for validating accuracy, functionality, and ease of use.