Here is FBI homicide data by weapon for 2014:
In 2014, 248 people were killed by rifles. Rifles would include “assault weapons”. Thus, more people are killed by knives than by assault weapons. Indeed, more than twice as many people are killed by “hands, fists, feet, etc.” than by assault weapons. (Some of these numbers could change slightly with “Firearms, type not stated” although most of these are probably handguns).
The data may be uncomfortable to both left and the right. The left because banning “rifles” would obviously not save many lives even if one assumed no substitution effect towards other weapons and banning “assault weapons”, however defined, would do even less. The right because handguns are by far the primary weapon used to kill.