the Bushmaster AR-15 that they are calling an assault rifle, is not actually an assault rifle, but a mid length semi-automatic rifle. An assault rifle, must use an intermediate cartridge, must have a detachable magazine, and must have a selectable rate of fire. As in you have to be able to switch between semi-automatic fire (the gun fires every time you pull the trigger) and automatic fire (the gun will continue to fire when holding the trigger down, at least be able to fire a 3 round burst)
Now, lets look at the M-16, the US militarys assault rifle. It uses an intermediate cartridge, the .223. It has a detachable magazine, and it has a selectable rate of fire, semi-automatic, and 3 round burst (each time you pull the trigger it fires 3 rounds)
Now, lets look at the m-4. Again, it uses the .223 and has a detachable magazine. It has a selectable rate of fire, between semi-auto, and full auto (no burst, it will fire as long as you hold the trigger down, until the magazine is empty) Both the M-16 and the M-4 are true assault rifles.
Now, lets look at the Bushmaster AR-15 used in the new town shooting. It uses the same frame as the m4 and the m16. It uses the same round-the .223. It also has a detachable magazine. However, it does not have the selectable rate of fire, it only has semi automatic fire, which means each time you pull the trigger, you fire only a single round. to fire again, you have to release the trigger, and pull it again. You can only fire as fast as you can work that trigger-which is actually pretty fast, the .223 round doesnt kick that bad, so it does fire rapidly. It is an assault style weapon-but it is not a true assault rifle.