Here's what I understand as to how he got the weapons inside:
He parked his car at the back of the movie theater next to an exit door. His weapons and body armor were in the car.
He went around to the front of the theater and purchased a ticket. He took a seat on or near the front row.
After the movie started, he got up and went out the exit door. He propped the exit door open with something so it wouldn't close behind him.
He went to his car, kitted up, and armed himself.
He walked back in the exit door and opened fire.
I see nothing in the world that could have prevented this. He was meticulous in his planning. He was determined and committed to his course of action.