The Black Widow Volt auto rifle is a prototypical energy weapon developed by the Brotherhood of Nod.
Based on data and technology recovered from a crashed alien vessel (possibly Scrin) it is essentially a lightning bolt gun, firing massive concentrated beams of electricity, frying infantry to ashes. Vehicles don't stand a chance under prolonged fire either. It is lightweight and portable, but due to the difficulties of control, unshielded concentration chamber and exposed electricity, it is hazardous at best.
Unbelievably deadly and powerful, the Volt Auto Rifle could be called the best weapon in its class, although its lack of range makes it fall short of deserving the title.