I haven't clicked on the news yet this morning, but I think this picure helps explain things:

from the eye witness reports, I am surmising that he bought a ticket and then went in looking like a normal person.
A few minutes into the movie he acts like he receives a phone call, gets out of his seat, walks out the exit door, maybe rigs it so it doesn't latch behind, walks the 30 feet to his car. Once at his car he starts suiting up and gathering up his guns. Then he goes back through the same exit door. Throws a can/grenade or two of tear gase into the crowded theater and proceeds to shooting, first with the shotgun, when that runs dry, then he switches over to the AR which if you look closely at the pic above has a normal GI style 30 round mag still in it, not a drum mag or a beta mag which has been reported (errr.... Fabricated???... To push an agenda)
I think we can all imagine the degree of panic that set in, especiall with everbody running for the door.
So he either runs out of ammo or figures the police are on their way, and walks out to the exit door, and is about to get in his car. Supposedly, one or two cops see him but they think he is a cop just like them.
In other pics or footage, you can see the pieces of body armor and helmet lying on the sidewalk kind of close to the car.