While I will defend your right to carry a firearm, I also believe one must use some common sense. 

This was best illustrated a few years ago when a young black man was going to buy a BB gun that resembled a dreaded AR-15 . A local shopper freaked out and called 911; reporting lucid details that made the police believe that there was a mass shooting in process. Two officers ran into the store and killed the kid. Only then was it discovered that the gun was not a firearm, and the caller was making stuff up to get a reaction from the dispatcher.

There were mistakes made by all parties involved. This retailer now requires a trained sales associate to escort the customer and gun out the door. (The legal arm firearm owner should let the courts settle this and your carry rights. Follow all police orders.) If you're still thinking of toting an AR-15 inside a store, think twice. It’s your right but you must understand the consequences, if any.

My thought, if you see someone toting such a gun in the store, notify the manager as you leave the store. Call 911.Be a good witness and remember, avoid, escape and defend.