Banning bump stocks will have almost no effect on the lethality or frequency of mass shootings in America.
Firstly, they are not even that functional. They often lead to the weapon jamming or misfiring, and significantly reduce the accuracy of the weapon. In fact, not using a bump stock would probably lead to more deaths in the event of a mass shooting.
Secondly, if someone has decided to kill a lot of people in a mass shooting, the lack of availability of bump stocks will not change their mind. There will be just as many mass shootings, involving just as many killers, who will kill exactly the same number of victims, if not more.