Luger

The Luger is a bullet-type weapon that is extremely powerful, being able to kill essentially any target in a single shot, but will cost your maximum bullet ammo capacity to fire and will disappear immediately when said ammo is depleted. If you manage to fire the weapon without depleting it's ammo, it will instead have a reload time approximately the length of a late 70's era animated science-fantasy film. The weapon begins appearing on 7-3. 

Note that the cost of firing the weapon is affected by Back Muscle; without the mutation, the cost of firing is 255, but with Back Muscle, it is instead 555.