In her letter, "Explain to me the need for assault rifles," Dorothy Heieie asked why anybody needs a gun that can shoot more than one bullet at a time.
A gun that can shoot more than one bullet at a time per pull of the trigger is defined by law as a "machine gun." In the last 85 years, legally owned machine guns have been used in a grand total of 3 murders.
An "assault rifle" specifically is a subset of machine guns and is defined as a machine gun that is chambered in an "intermediate cartridge," a type of bullet that is designed to be relatively weak (at least compared to other rifles) so that the recoil is easier to control. A legally owned assault rifle has never been used in any murder in the U.S. ever, certainly not any massacre.
The guns up for debate are legally called "assault weapons." This class of guns is completely unrelated to the term "assault rifle." These types of guns are defined by explicitly listed features, most notably a pistol grip, a collapsing stock, and a flash hider. These features are mostly cosmetic and people want to ban them because they look scary.
The guns up for debate are NOT defined by rate of fire, caliber, magazine capacity, or any other characteristic related to function.
Sorry for any confusion.