That's because it's a 2048 texture reduced to something smaller than 512, especially considering all the detail from the normal map are tiny things such as roof tiling.
If you will never be close to the gun and kept it at such a low tri count there is no reason for a 2048x2048 map, MAYBE 1024 but could probably sit well with a 512 with no problem.
I thought dpi was just something relevant for printing? If your not gonna print it, it dont matter, all that matters is the resolution(512, 256, 1024 etc).