nice. I propsed a scrath build a few years back ( likely for the x25 ) i went so far as to buy a end mill to do the bolt face and a few other bits of tooling. ther eis now reason a blow back could not be done with out a kit with a few original pieces like the dust cover, and other cheap essy to get original parts. t the whole build if you have a mill could be done for like $100 with no kit less if you are really reasorceful' I have always felt a blow back bolt/carrer assembly would sell if it was under $75. it could be one piece with a screw in guide rod/piston. the reciver could be a standard AK or a roll your own. when you make that trunion make a good place with some ,meat to screw a optics rail in place.
Your set up could be used for every thing from a 17 hmr on up to most any pistol calibur with just a few easy to obtain pieces. make the carrer so weight can be added or adjusted for various caliburs.