Continuity mistake: When Chris is in the trailer with Tracy, he walks inside with his towel around the back of his neck. During the scene he takes the towel off, then when we see a shot from behind him later, the towel is still there.
Continuity mistake: In the scene where Chris is doing sit-ups outside the trailer at the slabs, the first closeup shot of Rainey, he's about to take a drag from his cigarette, but in the next shot, his hand is nowhere near his mouth. (01:35:40)
Answer: Having read Krakauers book: The rifle was a .22 Remington scoped semiautomatic. Accounts at the time mention that this may not have been the best choice, and it is clear that McCandless did not have enough experience to live in the wild, although it is impressing enough that he managed to survive for 16 weeks.