After the phase out of the N80 in 2051, the N99 sidearm became standard issue military gear. The N99 was known for its ability to survive in the harshest of conditions and did so in many tests, as 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 extreme conditions following the War, the N99 is commonly able to be easily restored to working order by anyone with even 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 many units are available for replacement parts.
Colonel Autumn's 10mm pistol can fire a total of about 577 rounds, the equivalent of 49 reloads, from full condition before breaking, roughly 23 more rounds than the normal 10mm. It fires around 20 magazines (or 245 rounds) more than the silenced version.
Silenced 10mm pistol - While not unique, the silenced 10mm pistol is a suppressed version of this weapon. It's weaker, but firing while sneaking won't give away your position.
Sim version Operation: Anchorage (add-on) - 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.
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.
End of information based on Fallout 3 cut content.
Sonora Cruz' 10mm pistol - a unique pistol carried by Sonora Cruz. It deals a whopping 100 damage, but Sonora's conficence is marked as "cowardly", so she flees from enemies and never uses her pistol.
One must face directly towards the body with the cursor directly on the body in first person view. Once this is done, hold down the button that changes the perspective and it should be possible to rotate the camera. Rotate it 180 degrees and you should see a "Search Colonel Autumn" message. You can also search Dad and possibly everyone in the room if they fall close enough to the wall. If it is not taken then, it will not spawn on him during the final fight during Take it Back!, being replaced by Colonel Autumn's laser pistol, or Colonel Autumn's laser pistol (Broken Steel) if you have Broken Steel installed.