Pay close attention to the scene where Billy Bob is at the field with his shotgun. Five rounds are fired with this gun. That’s OK because it is a standard pump shotgun, which probably holds five shells with the plug removed.

The slip-up is that after Billy Bob fires the fourth shot he pumps it, loading the last round into the chamber. Then he hands it to Mox who pumps it again before firing it. The shell we see ejected should be the last live round in the gun.