[@Shoryu]

There are some pretty close to the entrance. Also, he could have stolen it from a Park Ranger storehouse. Like most police units they have a SWAT Team and those rifles are popular with snipers for being able to shoot out windows or take out car drivers and whatnot. 

Or ya know, it's possible a piece of Earth which had a Gunstore on it landed nearby him when it was put back together haphazardly.



[@aladdin_sane] I did the novelty thing because I figured a real one would bother people. If y'all are cool with him finding a legitmate one I'd totally change that so be had it xD