I love traveling-time stories :)
His gun might not even work properly with little oxygen. The velocity may be a lot lower and would make it difficult for him to shoot accurately. If the oxygen is so low that he needs to wear a mask (or maybe the air has poisonous gasses, or both) that would definitely effect the bullets speed and angle drop.
i could see the ending coming a mile away but ill give 3 stars for the comedy factor
That was a wacky ass good time. You would have thought he would've been more prepared to take evasive action. Twilight Zone in idiot mode.