You DO NOT need to report to the police...its not stolen!!! If you know where you purchased it, they have records...they must keep them indefinetly. Or call local/reputable gun store and ask them what you need to do. Or if you have a relationship with a store you do biz with. An a AR-15 IS NOT ILLEGAL TO OWN OR HAVE IN YOUR POSESSION.
Speak to the licensing agency. I know that several communities have registration regulations. Sometimes the owner sometimes the firearm. Here in New York city, it's both. PS Here they have banned AR's. They apparently are evil and jump out of their storage place and kill people.