Barrel Shroud

AKA handguard aka forend. The anti-gunners say this is an "assault weapon" feature. It's not. The forend has been utilized since before the flintlock. The AR-15 uses a gas tube, seen running along the top of the barrel, which channels gas from the barrel back to the bolt carrier group to cycle the action. The handguard goes all the way around the barrel to protect the gas tube as much as to provide a handhold. It's that simple.