The FBI has released an interesting chart plotting homicide numbers over the last five years. (the declines would probably be a little sharper if rates were used, since the population increased over that period of time).
Total homicides: down from 14,916 to 12,664, a drop of 15%.
Firearm homicides: down from 10,129 to 8,583, a drop of the same.
Rifle homicides: down from 453 to 323, a drop of 29%.
Rifle homicides (of which "assault rifles" are but a part), are now far below homicides by club or other blunt object (496), or by beating (728) and far below those committed with knifes (1,694).
Hat tip to Prof. Raymond Kessler.