It lobs missiles like a grenade launcher, so it has a short range, but due to the fact that it doesn't use the rocket fuel in it, it has an extra kick in the explosion.
To get AC-13's Plasma Rifle, complete the quest "The Replicated Man" by finding out who the Android is. Confront him (Harkness, the Security Chief of Rivet City) and if you have all the infromation (The Proof and override code) you can give him his memories back. Ask him what he'll do next, and he'll tell you that he wants to kill Dr. Zimmer. Offer to kill Zimmer for him, and he'll give you AC-13's Plasma Rifle and permission to kill ONLY Dr. Zimmer and his android buddy. (NOTE: Once you find them, rather than kill them right away, tell Zimmer that Harkness is android AC-13. You'll get bad Karma, which should balance out with the good karma you just got, and the Wired Reflexes Perk, which ups your accuracy in VATS. Once your conversation with Zimmer is over, immediatly kill him and his Android companion. No one else in Rivet City will become hostile, and you're walking out with an awesome Plasma Rifle and a great new Perk)
Wanna listen to me rant about stuff? I started a blog!
https://theloonybinblog.wordpress.com/