A Wattz 1000 laser pistol. Civilian model, so the wattage is lower than military or police versions. The reason for this is that it uses a laser focusing diode instead of a crystal array. For security, it can be locked down for transport, with a proper arming sequence needed to render it operable again.
Laser weapons tend to become less effective as the game progresses due to the fact that most late-game enemies have high resistance and threshold to laser damage. The Wattz 1000 can roast most weaker enemies, but you'd be better off with a Glock 86 if you want an energy weapon handgun.
In the game, the Salvatores have begun conducting secret transactions with the Enclave (the player will be asked to guard one such transaction for Louis Salvatore's final quest), trading dangerous chemicals for Wattz 1000s (or "lightbringers" as they are referred to by denizens of New Reno). This gives the Salvatores an immense advantage over the other "families", and, using the "lightbringers", the Salvatores brutally massacre Mordino family members in the streets, striking fear into all those who oppose Salvatore.