Actually, “chain gun” refers to the system used to cycle the weapon. Instead of using blow-back or gas-operation to cycle the bolt, it used an external electric motor connected to the bolt with a chain similar to a bicycle chain. It has the advantage of continuous operation even if a round fails to fire. Another advantage is that it allows the operator to easily select the appropriate rate-of-fire. (Also, a chain gun shouldn’t be confused with a Gatling gun. Same principle and advantages, but a chain gun only requires a single barrel.)