The large-framed, gas-operated N99 was selected as the standard issue conventional sidearm of the United Statesmilitary, following the phaseout of the N80 in 2051. The long-barreled pistol remained in service for 26 years, until the Great War destroyed the United States. The pistol is designed to withstand the most extreme conditions, which was one of the requirements for a new service sidearm. It can also be repaired easily by anyone with even the most basic technical knowledge. This includes both clearing misfeeds and restoring a unit to working condition after long exposure to elements without maintenance.
These features also make it popular with wastelanders, who appreciate the ease of maintenance. It remains a common weapon, as even though many civilian units were ruined, they can easily be restored by anyone with some repair knowledge. Even damaged units can be used reliably, though they will offer subpar performance and jam frequently. The number of manufactured pistols means that parts are easy to come by, however.
An N99 10mm pistol equipped with a suppressor, which allows for silent kills if the target is killed with the first shot. This stealth feature comes at the cost of durability and damage, however a sneak attack always result in a critical hit, so the lower damage can be compensated for, especially with the Better Criticals perk.
Firing the silenced 10mm pistol will not increase your chance to be detected, unless the player has a very low sneak skill. It is therefore useful for killing weaker enemies like radroaches, mole rats, and sometimes even raiders, without drawing attention from other enemies. A high enough Small Guns skill makes it a useful close-quarters weapon in office building type settings, such as the Bethesda ruins and the Museum of History.
Sim version - A sim-only version of the weapon which has been re-skinned to appear newer and less worn. It also differs in health, with a whopping 999,150 hit points, essentially preventing the gun from ever degrading, This weapon is not normally accessible outside of the simulation.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
No ammo silenced 10mm pistol - This strange pistol is only obtainable with the Operation: Anchorage add-on. This variant degrades but has a bottomless magazine.
10mm alloy steel pistol - Another version of the weapon was cut from the Operation Anchorage add-on, it differs from the standard 10mm pistol in appearance only, using the same new skin as the sim only version.
Sonora Cruz' 10mm pistol - a unique pistol found on Sonora Cruz. It deals a whopping 100 damage, but Sonora always flees from enemies, and never uses her pistol.
End of information based on Fallout 3 cut content.
In the caves under Oasis, after you decide the fate of Harold, head to the door to your right, there is a cave where you can find it. You cannot go back, but you can dive into the nearby water and exit via the underwater door back to Oasis.
Underwater by a sunken car at the docks near Arefu, along with an ammunition box with 10mm rounds.
A pre-war variant will be given to player at the beginning of the Anchorage Reclamation simulation, and will be removed after you take out the Chinese Artilleries.
The no ammo variant can be found on a table along with other cut items in a room next to the one you teleport in by clipping into it after teleporting into the TestPhil_(Operation:_Anchorage) room via console commands.
↑The Lone Wanderer: "Your gun is jammed? Can I take a look at it?" Pek: "Sure, I guess. Can you fix it?" The Lone Wanderer: "Sure can. Here, just pull this back to clear the chamber." Pek: "Wow... that's... that's easy. Now at least I can shoot back at those mutant bastards. Are you ready?" (FFDCFallsChurchBoSLostInititate.txt)
↑ 2.02.1Citadel terminal entries: N99 10mm Pistol: "The 10mm N99 sidearm was standard issue military gear following the phase out of the N80 in 2051. The N99 was known for its ability to survive in the harshest of conditions and in many tests, the weapon was able to reliably be restored to working condition after an extended period of time exposed to hard elements and without maintenance. The N99 proved to stand up to its claims and as a result, is one of the most common weapons found in use in the Wasteland. While many civilian weapons were ruined beyond use in the exterior conditions following the war, the N99 is commonly able to be easily restored to working order by anyone with the most basic knowledge of firearm mechanics. However, an N99 in poor condition can often deliver low target and armor penetration, frequent jamming, and greatly reduced accuracy. Fortunately, the weapon was produced in great numbers and units are available for replacement parts."