Well I'm glad you asked.
AR15's are not commonly capable of full auto operation so your premise that its a weapon used by the military is not correct.
The US Constitution does not limit THE RIGHT TO BEAR ARMS by civilians to any particular types of weapons.
Finally, I have never, even once, ever heard of a good guy using any gun in any mass shooting. Only bad guys aka criminals.
So, to turn your question around....for what possible reason would you want to say GOOD GUYS should be limited as to what weapons they can own?
So why try to limit the good guys if they dint do nuffin???
Seems you should support tougher laws against CRIMINALS. But unfortunately the Left has a long history of collusion with and protection of violent criminals. NY and CA dont even want to hold murderers in jail. If you were sincere about public safety you would focus your efforts there
(And a law that to get any hunting license you must bag at least one other hunter each season)