Basically, there's a funny enemy in this game called a "war robot", you only really see this guy in military camps (areas with a white tent logo on the map) basically just kill this guy and you'll get the "high energy laser rifle" and the "tesla gun". It's a hard fight but if you can somehow get the robot stuck in one of the tents like I did, then you'll be granted some real good weapons. The laser rifle does 600 damage over time, tesla gun does 200 or 400 I believe.

The gun mastery skills don't seem to do anything to them, but they are seemingly the best guns in the game. The tesla gun acts as a mining weapon mostly as it can basically mine through anything.

Have fun