Hollis vs. Holder and Watson vs. Holder should get real interesting soon. ATF messed up by approving new Form 1 machineguns, then revoked the stamps and confiscated the weapons. These suits (and more along the way) have the potential to unravel the NFA, GCA, and 922(o).
There is also the state level route, by passing laws that protect intrastate manufactured arms, states can avoid Federal regs.
Watch liberal heads explode if the entire chapter 4 of GCA is struck down as unconstitutional.