Heres mine. it can be either a rifle or a pistole. I've only worked out the mechanics so hear goes.
imagine a cilinder with a glowing crystal inside it, the inside of the cylinder has mirrors inside it angled toward a small circular opening in one end. Over the opening are several lenses that magnify the beam. put that in any type of gun, its the same concept as lighting a fire with a magnifying glass and the Sun.
Sounds like a weapon that would get more destructive the longer you shot it.