Yes you can shorten it all you want. I think mine is 10 inches. But if you do much modification and create a short barreled rifle, then there is a problem. So, just do not put a stock over 13.5 inches on it and no problem. The 13.5 inch rule came from some ATF advisory, and while not hard law, to my knowledge, at some length a butt stock is a butt stock, so stay away from that fiasco. Not worth the hassle.
And of course do not put any type of vertical grip on the front of the gun, as one on a pistol creates and "any other weapon" category and requires a tax stamp and all that. https://blog.princelaw.com/2019/07/0...ilizing-brace/
And for your info, 10 inches has traditionally been known as the perfect or most efficient barrel length for a 22 LR. If you put a shorter barrel you lose velocity, a longer one you do not gain much. Here is a link. BBTI - Ballistics by the Inch :: .22 Results
. I have measured a lot of them and this link is very close to my results.
I shoot my take down charger suppressed and have 4 semi auto 22 pistols I use with the same suppressor. You just lose way too much velocity with say a 5 inch barrel, which of course cuts your effective range way down also.
A couple of useful links: https://www.offthegridnews.com/self-...u-not-know-it/
Hope that helps.