Other mistake: The closing captions state that Chris McCandless' sister Carine flew "his" brother's ashes home, instead of "her" brother's ashes.
Other mistake: When Chris is sleeping in his yellow car during the thunderstorm, he gets hit by a flash flood. Afterwards he abandons his car and removes his licence plates. His plates are: UW4-924. A few minutes later his parents decide to visit Chris in Atlanta. We then see a shot of a white car on the freeway, which has the exact same licence plate (UW4-924).
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.