The 10mm N80 sidearm was standard issue military gear until it was phased out in 2051.
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.
A N99 10mm pistol that has been outfitted with a military-grade suppressor. This modification comes with the benefit of not alerting nearby enemies when the pistol is discharged. The downside, however, is the suppressor results in slightly decreased damage compared to a non-modified 10mm pistol. If a target is not taken down quickly enough, nearby enemies will still be alerted.
The suppressed pistol counts as a silent weapon for sneaking purposes, and shooting it will not increase the chances of being detected.
Sim version (Operation: Anchorageadd-on), a sim-only version of the weapon which differs only in health (essentially non-degrading) and appearance. It appears grayish-black, has what appears to be a wood grip, and looks less worn out and old than normal silenced 10mm pistols do.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
Silenced 10mm alloy steel pistol (Operation: Anchorageadd-on), a normal health version of the sim version that was cut from the final release of the add-on.
End of information based on Fallout 3 cut content.
↑Fallout 4 loading screen hints: "Small, dependable, reasonably powerful and widely available, the 10mm Pistol has been a staple of Wasteland combat since the bombs first fell."
↑ 3.03.13.2Citadel 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."
↑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?" (Pek's dialogue)