The AH-64 Apache is heavily armored. The rotor blades themselves are designed to withstand several hits of 23mm and not break.
the Mi-24 (NATO "Hind") helicopter is even tougher. It's WINDSHIELD is rated to stop .50BMG, from what I understand, and there are no exposed components that are vulnerable to small arms fire.
In other words, an M16 or a Kalashnikov of any variety isn't going to cut it against a dedicated attack helicopter.
Hueys, Mi-8s, Blackhawks, Bell Jetrangers, Lynxs, etc. are different stories, however. The older ones are made of aluminum, the newer ones are made of aluminum and fiberglass.