if you find an avian sky ship (you will see anchor on the surface of the planet ) the quarter masters sell guns
Concerning the guns, not alot of stuff changes from them except special effects. The intensity of these special effects get higher the rarer the weapon is. So far my favourite gun is that one: Some sort of elephant rifle with a slow projectile that goes through ennemies, hugs the first wall it collides and keeps going. Can damage ennemies multiple times with the same shots. Oh, and there was that cluster grenade launcher doing massive damage if you hit with the big projectile.