Devil's Advocate Mode On
Bump stocks, arm braces, and 80% Kits could be viewed as walking a thin line between legal definitions and could be seen as an act, from the ATF's POV, of poking the bear. Well the bear got tired of being poked.
Not taking the ATF's side here just stating what may be their viewpoint on this for discussion purposes so don't flame me.
Devil's Advocate Mode Off
Now that said. It's legal or it's not. Don't go saying it's legal one day and then the next say you changed your mind so turn it over. The ATF has more mood swings than a feminist convention on the rag and that has to stop.