wood a dedicated 22 upper be restricted if i used a shorter then 18.6 on it id like it to be a do most rifle system
IANAL so don't take this as legal advice, but here is my understanding of the law:
Rimfire rifles only have an overall length restriction. Pretend you use an A2 stock for it - then all you need to do is make sure the barrel is long enough to meet the OAL requirement of 660mm.
If you go with a collapsable stock, you need to make sure the barrel is long enough to meet 660 mm in OAL at its most collapsed state (which means a longer barrel than the A2 stock example, obviously). It's also nice that there are no mag restrictions so if you get the CMMG or that type of rimfire adapter for it, you can run those 24 or 26 rounder Black Dog Machine mags with it