911 What is your emergency?
There is a man with a gun walking down the street!
Is he threatening anyone, being hostile in anyway, or confronting anyone is particular?
No, he is just walking around.
We will send an officer to observe him. Open carry is legal and many of these people are just trying to get attention. We are going to handle this by not giving him any direct attention unless he is breaking the law.
This would be in a perfect world, but after you get 50 calls from the public and no cops show up, the police will be accused of not doing their jobs.
I understand carrying a gun in a large city when walking down dark alleys at night, but you HAVE to agknowledge that a guy walking around with an AR15 in broad daylight is very suspicious. A good citizen reports something like that. What if the guy in that school shooting would have been walking down the street towards the school with his holstered pistols and his AR slung around his back? With your example 911 call, he would have never even been questioned until too late.
My point is that although it is legal, it is highly unusual. It would take a horrendous police force and apathetic citizens to allow a guy to walk up and down a street with that weaponry without being questioned. That leads to police having to take the time to go question him.
...and we're back to square one. If he is just doing it for fun, then he's an inspection sticker checkpoint.