From WikiGTA - The Complete Grand Theft Auto Walkthrough
The electrogun is a weapon that emits an electrical charge. Contrary to normal weapons, your health won't be reduced when hit by the electrogun. However, if you're continuously in the beam for several seconds, you will die instantly, no matter how much health you had left.
The weapon automatically aims for anyone in front of it and may hit several targets at once. The charge can also 'jump' between targets, increasing its reach. The electrogun is even capable of killing someone with invulnerability*. NPCs killed with a electrogun will not leave their weapon behind.
- The weapon can be seen on artwork of cops arresting someone. The cops ingame do not use the eloctrogun, presumably because it's very powerfull.