When Tyler and Clay are shooting, the police show up just as Clay reaches the end of his magazine as evidenced by the slide going fully back. Though he and Tyler flee the scene, in the next segment with Clay has the gun and the slide is back in it's normal position. He's never been shown how to load the gun and it's not suggested that he stopped to buy more ammo. See more
[when Tyler plans to shoot up the school dance
Tyler, listen. You don't get out of this alive. And I don't want you to die. I... I don't want you to die. If you think this is the way, if you really think this'll change a goddamn thing and not just be another fucking tragedy that adults cry about for a week and then forget, if you really think this is gonna be different, then do what you gotta do. Come on, Tyler. Please?
There's nothing else I can do.
There is. We'll figure it out. Okay? I swear.