I don't remember much: a guy with a big gun/cannon attached to his hand (his hand was inside the tube). Cannon wasn't an ordinary gun, I guess he found it in some space ship, I'm not sure. I remember a scene when guy is walking through some kind of desert, sunny place. And a last thing: the cannon had a bad influence on the guy. He wasn't feeling good for sure.
Was it Laserblast, (1978)?