This is my opinion and only mine..
But why we're you putting yourself out there to the cops to be stopped in the first place? What was the reason for even having your ar15 at the time?
Yes your within your legal rights, but I don't see the necessity of it, your just asking to be stopped, questioned and harassed.
"Sent via the Skyrim messenger service"
No offense, but its sort of speaking with a forked tongue to say its a right, but then question the necessity of it. If one genuinely and fully supports a certain right, necessity doesn't enter into it. The only person who gets to determine the necessity is the fellow to whom the right belongs.
Was he testing the police? Maybe. But, even that is totally fair. There is no reason whatsoever citizens cannot test and verify that government respects rights and uses only the authority it has. In fact, citizens should pay lots more attention to government and give it lots more oversight.
Was he just making a statement? Maybe. Still totally fair. To my mind, OCing an AR is just as protected as OCing a handgun. OCing an AR may be highly uncommon, but it is not government's place to say whether it is OK. It is not government's place to say it is suspicious. Government has no legitimate say in it at all until he starts to give objective signals that he is going commit a crime with it.
Rights are rights are rights are rights. We would all do well to examine the subject of rights in some detail, including what it cost to get them, what it cost to maintain them, and government's activity regarding them. How else can we defend and keep them if we don't really understand them and their value.