The YK32 Pulse Pistol is an electrical pulse weapon that was developed by the Yuma Flats Energy Consortium. Though powerful, the YK32 was never considered a practical weapon due to its inefficient energy usage and bulky design. The YK42B Pulse Rifle is a similar, but more advanced weapon.
You need to complete the Newton mission to unlock this weapon. It is possible you also need to complete the optional objective there, which consists in taking a prototype of pulse gun. Then the weapon becomes available in several bunkers, including Bunker Delta and Bunker Epsilon.