This is why all of us should be building SBRs, getting "unregistered suppressors" and sawing the barrels off shotguns continuously for the next 3 months.
Remember - the NFA was modified by Congress once in the past because they fucked up and sold a bunch of surplus M1 Carbines that violated the original 18" barrel length. That's why it was 16" for rifles and 18" for shotguns.
They couldn't possibly claw back all of those carbines, so they gave up.
Make them give up, again. This time once and for all. We have the window; all we need is the will.