To make it as short as possible, the definition of an automatic firearm is one that fires multiple rounds with a single pull of the trigger. The ATF, originally and under current law, issued an interpretation that bump stocks did not alter a weapon that would make fire multiple rounds with a single pull of the trigger. Is your breath still bated (whatever that is)? A 1 minute internet search will verify this.
http://www.slidefire.com/files/BATFE.pdf
“The ATF, originally and under current law, issued an interpretation that bump stocks did not alter a weapon that would make fire multiple rounds with a single pull of the trigger.”
I bet all those folks killed at Las Vegas by a psycho firing semiautomatic weapons that were made to fire as automatic weapons feel much better now that that is cleared up.