I've been thinking about this for a while, and I've seen others mention it too. If we assembled an upper with a dedicated rimfire barrel, as long as the assembled package stayed over 26" long, we would stay non restricted, and be able to plink with full capacity rimfire magazines. I'd love to build a Mk18 Mod 1"ish" upper.
What would be the best rimfire bolt kit? Is there one that uses decent magazines and has a last round hold open that uses the bolt catch? ATRS, can you guys provide the barrel for a project like this?