Section 5845(b) of the NFA defines “machinegun” as “any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger.
Nothing about a bump stock changes what a single function of the trigger does on a semi automatic firearm.
Ah yes, it's coming back to me now. It's been a while since this was being discussed (and I was reading).