That Gun is a heavily modified, pared down .223 pistol, successor to the same weapon from Fallout and Fallout 2. It weighs a hefty 5 pounds and has a motorized cylinder and crane for quicker shooting and reloading than a typical revolver. The green light on the left side turns yellow while reloading, then turns green and makes a ringing noise once the weapon has reloaded. Until the advent of the add-onGun Runners' Arsenal, it had the unique capability of using dedicated armor piercing rounds, something that no other pistol in the game was capable of. This gun is also the least expensive named rare gun that is available for purchase in Fallout: New Vegas.
Along with Lucky, it has the second highest potential critical hit chance (x2.5)  of all one-handed non-energy guns, right after the silenced .22 pistol.
It's located in Novac, inside the Dino Bite gift shop. It can either be purchased from Cliff Briscoe, or stolen from the storage room. It is on the end of the first shelf with the Rocket souvenirs to the right. The storage room key can be pick-pocketed or the lock may be picked (very easy). The key may also be purchased during the quest Come Fly With Me or any time after its completion.
According to Joshua Sawyer, its name is derived from the fact that many players referred to the .223 pistol as "That Gun in Fallout and Fallout 2."
Like the .223 pistol, That Gun is modeled on the pistol used by Rick Deckard, the main character of the film Blade Runner. This version differs considerably in proportions, and lacks the bolt handle, ejection port, and second trigger.
pcps3 If stolen That Gun from the storage room, Cliff Briscoe still sells it in his shop, creating the possibility to obtain a second weapon. [verified]
pcps3xbox360 If loading a saved game from the main menu in which That Gun is equipped and drawn, the reload animation takes twice as long as normal. This can be solved by simply switching to another gun and then back again. [verified]
xbox360 Rarely, when aiming down the sights of That Gun, when turning right or left the camera will instead pivot from the Courier's shoulders, but the gun will stay in the same spot, causing it to careen to the right and left. One's head can even be moved backwards, to the point of seeing the viewmodel hands. [verified]