The Energie Gewehr 42, or EG42, rifle was a very early form of energy rifle, used in a very limited form by the Germans in World War II. It was designed by Friedrich von Stauffenburg and some of his colleagues, as part of a secret German wunderwaffe program.
It was never deployed, and only three prototypes were uncovered by American OSS operatives, alongside several hundred pages (many of which were partially ruined by fire). The weapons, and the notes, would later form the basis of both the M18 rifle, as well as the AER series of rifles.
The EG42 was developed by a very small team of just five men. At the head of this team was one researcher known as Friedrich von Stauffenburg, who was a nuclear physicist with a firm grasp over many of the principles of nuclear physics. Making use of finds from the heavy water research project in Norway, Stauffenburg and his team developed a laser rifle -- or rather, a radiation rifle, which took a charged bit of uranium and, by use of a mirror array, concentrated this force into a single point. A lead casing protected the researchers from the radioactive material, while a retractable "iris" essentially allowed the rifle to turn on or off.
The "projectile" was invisible, and was a constant force for as long as the iris was open.
The EG42 was never employed in open combat, but rather was shelved toward the turning point of the war in favor of more traditional firearms, such as the StG-44 and 45. The few reports of the weapon's testing indicated that it would, after several seconds, begin to melt through flesh and bone, and through even greater exposure could melt through metal.