You may be confusing "assault rifle" and "assault weapon" which are different categories. The AR15 does usually qualify as an "assault weapon". The definition of assault weapon is looser, and includes a number of features (such as barrel shrouds and flash suppressors") the sole purpose of which is to make the gun safer to use, and have nothing to do with making them actually more dangerous.
The terms I am using are defined pretty clearly on Wikipedia:
https://en.m.wikipedia.org/wiki/Assault_rifle
https://en.m.wikipedia.org/wiki/Carbine
You may be confusing "assault rifle" and "assault weapon" which are different categories. The AR15 does usually qualify as an "assault weapon". The definition of assault weapon is looser, and includes a number of features (such as barrel shrouds and flash suppressors") the sole purpose of which is to make the gun safer to use, and have nothing to do with making them actually more dangerous.
https://en.m.wikipedia.org/wiki/Assault_weapon
If you have different sources, feel free so share.