Where does Reese get the extra shotgun ammo at the beginning of "The Terminator"?

Question

In the beginning of the original "Terminator" movie, Reese reaches into a squad car and grabs a pump shotgun with an extended magazine but no extra ammo. This shotgun holds, at most, eight or nine rounds. Yet he is able to fire more than that number of rounds in the TechNoir scene, plus he continues to fire multiple rounds in the chase scene, when he and the Terminator are shooting at each other.

Where did he get the extra rounds?

(Also, as a side note I must add when he is seen reloading they are all low-base rounds. Police loads, either buckshot or slug, would almost always use high-base rounds, i.e., a taller brass base)

Answer 1

The film's official novelisation explains this. After he arrived in the past, he had a few hours to kill while he was waiting to acquire Sarah. During that time he helped himself to a car, then broke into a sporting goods store to get hold of some ammo for the .38 handgun he took from the policeman and the shotgun he purloined from the cruiser.

Reese sank into the seat and leaned his head back. He felt his muscles, knotted by tension and use, begging to relax. A wave rolled up to the shore of his conscious mind and offered to carry him to a warm, peaceful place for a few hours’ sleep.

No way. He sat up and emptied his pockets; three boxes of Super .38 ammo and four more of .00 buckshot. That would hold him for the time being. He had broken into a sporting-goods store at 9:15, and by 9:16 his pockets were full, and the guard dog there was sorry to see him go.

_{^{Terminator: Official Novelisation}}

Notably, he also got hold of a whole bunch more .38 ammo while Sarah was sleeping after their escape from the Terminator.

“What have we got here? Corn syrup, ammonia, mothballs. Umm. What’s for dinner?” Reese didn’t rise to the limp attempt at humor. He was unpacking another sack filled with ammunition for the .38, road flares, tape, scissors, a small pan with a strainer, and matches. “Plastique,” he answered, distracted.