Here's a weapon that John would like
It's the Pandora from Devil May Cry 4. It's a Devil Arm that is in the form of a briefcase and has 666 forms, only 6 of which are used in the game.

The forms in order of which they are used.
PF262: Jealousy: A stationary Gatling gun.
PF124: Hatred: A rocket launcher that fires a group of 3 rockets at once
PF422: Grief: A spinning blade that is thrown and returns back to the user after being thrown.
PF594: Argument: A mobile Missile Battery that fires a barrage of missiles (good for defensive purposes.)
Then their is it's ultimate move.
PF666: Omen: Open Pandora's box and damage all surrounding enemies with the flash of a violated taboo when the Disastor Gauge is at full (it's energy that is charged when using it's other forms.)
Other forms that are not in the video:PF013: Epidemic:

A large bow-gun that fires explosive shots
PF398: Revenge:

Transform the Pandora into a laser cannon that fires a searing hot laser to decimate foes!
Since it is not a actual gun, but rather a Devil Arm it does not use ammo. Instead the Pandora uses something called the Disaster Gauge.
It fills very slowly over time in this RP, as well as landing successful attacks with the:
Epidemic, Revenge, Hatred, and Jealousy forms. (Landing successful hits with these forms fills it much faster.)
The Argument and Grief forms use Disaster Gauge energy in order to fire, while in order to use the Omen form you need to have a full Disaster Gauge. Using Omen depletes all Disaster energy.