I don't think we need SBR/SBS/suppressors (and probably AOWs and most DDs) under NFA at all, but I'm kind of ok with at least making the requirements for select-fire consistent for individual vs. organization, especially after that LAPD Dorner guy.
However, if an armed citizenry is supposed to keep the government in check, why not HEAT warhead anti-armor weapons? E.g. in the context of this discussion: https://news.ycombinator.com/item?id=6186569 which touches on the widespread police procurement of light armor.