It's a port of omegaman's Advanced Alien Blaster mod for Fallout 3. I removed the head explode effect and changed it to use ordinary energy cells.
http://www.fallout3nexus.com/downloads/file.php?id=7604

Damage: 200x0.5
Critical Damage: 150 (And yes, that's per projectile)
Ammo: Small Energy Cells
Explode only

It's found floating around in Victor's shack in Goodsprings. You have to get your own ammo, though. This mod is cheat enough.