IF Aaron Alexis picked up the rifle and handgun “along the way”, I seriously doubt he had an AR15. I kinda doubt that military or POlice on a military base would be carrying the CIVILIAN model. My guess is, M-4. In fact I mentioned to my wife that when the black woman described the shooting said, she heard the shots “popopop”. She actually sounded like an auto fire AND only voiced three pops.
I thought the same thing.
Even then, in my experience, a loaded M16 is a rare weapon on a non-infantry, non-basic-training military base.