The 5.56mm high value pistol isn't the most powerful gun in Fallout New Vegas, but That Gun, the rare weapon version of it, is quite a bit more powerful. Watch this video to learn how to find and acquire That Gun.
From the Creator:
It's located in Novac, inside the Dino Bite Gift Shop. It can be purchased from Cliff Briscoe for around 1500 caps. With a barter skill of 20 'That Gun' can be purchased for '1337' caps or, if you prefer, stolen from the storage room.
Not the Best idea to kill the shopkeeper; He is a valuable source of information. Do not sell the rocket souvenirs they are quest related.