Machine guns were the only gun banned that the NRA was ok with at the time.
Too many Tommy gun sprays into crowds made it a serious safety concern for regular people and they knew letting them ban those would head off public sentiment to revoke or handicap entirety of the 2nd amendment...
my guess is if they are moving back in that direction, then it will lead to a larger attempt to totally remover the 2a in the near future.
My guess would be a moderate proposal to reign in extreme guns that is exploited for more than what was suggested once a constitutional convention is convened...
Slippery slopes and bad agendas, as always