I would have walked up behind him, tackled him to the ground and ground and pounded the shit out of him, and then yelled out, it's alright everyone, the GUN NUT has been apprehended. Maybe might even disarm him, and point the gun in his face, just to let him know what a douche-bag he is.
So, if I see a guy with an assault rifle heading into your kid's school, I'll just ignore it because it's obviously a law-abiding gun owner carrying an unloaded weapon. Problem solved! Thanks, dude!