The Rainbow Gun is a Legendary Lightning Gun who's regular attack functions as a beam rifle while not chaining to electrocuted enemies. The beam, as stated before, acts like a beam rifle, piercing through multiple enemies in a line in front of Victor and electrocuting them. Because it doesn't chain to enemies anymore, the regular attack never overheats (when the regular attack stops on a Lightning Gun after prolonged use). thus the attack button can be held down indefinitely. However, when releasing the attack button, the attack has a 10 millisecond cooldown period before you can use the beam again. Because the beam never ends, a Ball Lightning can hit an enemy much more often then it would with another Lightning Gun. Combine this with The Lightning Destiny Card, and you can have 5-6 Ball Lightning attacks on the field hitting a multitude of enemies for a much longer period than any other Lightning Gun in the game (untested but arguable). The gun seems to be apart of a set of Legionaries that include the Finger of Chaos and, potentially, the Chicken Canon as seemingly joke weapons or fan weapons with a rainbow like motif.