I've been looking into this for some time, however I won't have time to get such project done.
IMO best option for carbine would be: JR (modular system with detachable mag well and barrel) + custom barrel + custom mag well (3D printed?) and as for mags I would go with NP726 mags (because with other mags you're limited to 4-5 cartridges). It is also highly possible that JR 9mm bolt would extract 7.62 with no issues, as geometrically cartridge base dimensions are almost identical. 7.62x25 energy might require some experiments with springs of bolt weight, otherwise it's easy and doable. Hell, one can start making conversion kits.
Great idea on NP762 Mags. I will be looking into it. I was thinking of using my 1911 mags. I managed to put 9 factory rounds in Mec-gar. I would love to know the np762 mag dimensions. Does anyone have one and a caliper to do some measurements?


















































