whats the name of that book?

Maybe there's a better place for this, but here goes anyways. Guy finds a crashed space ship on a planet. In said ship he finds a weapon(gun). Also a robot, maybe with the gun, don't remember that well. At one point the gun shoots a hole through a planet. That's the most I can remember. I read this book in the late 80's early 90's. Something I remember about the cover, the gun was blueish with an orange protrusion that ended in a sphere. Plz help, been bugging me for years.