An AR-15 style rifle is not an assault rifle. The definition of an assault rifle is "a selective fire rifle that uses an intermediate cartridge and a detachable magazine". Selective fire means "the capability of a weapon to be adjusted to fire in semi-automatic, burst mode, and/or fully automatic firing mode". These were banned by the 1986 Firearm Owners Protection Act.
There are "assault weapon" bans like that in my home state of California that ban certain cosmetic features on guns. These are completely pointless as an AR-15 is functionally no different from any other semi-automatic firearm.
Examples:
Assault Rifle
https://i.imgur.com/pGnxHRD.jpg
"Assault Weapon"
https://i.imgur.com/xhsZ2dK.jpg
Not an "Assault Weapon"