sounds like you have built some cool stuff. It would be great if you could post some pictures of those projects.That's the trouble about being retired, having skills at hand, and an active imagination.
Small necked cartridges like the .17 Mach 2 or the .17 HMR on the small end up, to the five seven or the 5.56 or 5.45X39 beg to have "boy's guns" designed around them.
Practical?!? Who wants practical when there are so many fun projects out there.
Miniatures done so far are an M12 (a scaled down Mac 10 or 11 using the ratio between its .25 Auto cartridge and a .45ACP or 9mm.
A 106 Recoilless scaled to use a 12ga single shot barrel (that one was fun because of the interrupted thread breech block).
Both work and are real attention getters as a table-top display.
Now that I need glasses and tiny parts seem even tinier, I'm currently 80% through a full-sized "Commercial Model of 1885" Gatling in 45-70.
A boy's gotta have a hobby, right?
dont for get 22 reed a necked down 7.62x39 its a neat little round that would be fun in a small bolt action an it uses the same rim bolt size as a 9mm or 7.62x25.