The energy weapons in the Fallout video games have always been a mixed bag, but it's hard to argue with their raw destructive power if you can find them and keep them in ammo. Watch this video to learn where and how to find the Pulse Gun rare weapons energy weapon in Vault 34.
From the Creator:
Located inside the Vault 34 Armory in a very hard locked chest. The Armory door is initial locked, you must access the terminal in the Overseer's office to open it. It can be obtained with an Lockpick skill of 100, but a key to the chest can also be obtained through a companion (Veronica)'s loyalty quest.
There is a chance that it can be purchased from the Silver Rush in Freeside.
one can be looted from Fiends near Horowitz Farmstead.