Fallout: New Vegas weapon mods

Overview
One of the features added to New Vegas is the ability to custom modify certain base weapons. While in Fallout 3 this was limited to the creation of custom weapons, New Vegas takes this a step further by allowing the creation of weapon variants based on basic weapons by adding weapon mods.

Unique weapons cannot be equipped with weapon mods, with the exception of the Weathered 10mm pistol, Mercenary's grenade rifle and Holorifle. While unique guns usually have better base stats, Obsidian Entertainment introduced this inherent drawback of unique weapons to balance out unique and normal weapons and make the choice less of an obvious decision and more of a preference. However, some unique variants have both mods and better base stats, such is the case of the Varmint Rifle's unique variant, the Ratslayer, which has all of the mods the Varmint Rifle can be equipped with.

By themselves, the weapon mods serve no function but, once attached to their specified weapon, will change the characteristics, look and/or function to create a distinct variant. The weapon mods range from scopes and silencers to extended mags and several in-between. In the case of the Varmint Rifle and other such weapons there exists more than one different type of modification. While you are not limited in the amount of total mods you can apply to a single weapon, you are limited to one of each type of mod per single weapon and therefore are not able to stack attributes like increased rate of fire.

Once a weapon mod is attached to a weapon it cannot be removed and remains a permanent feature of that weapon, along with its added attribute. However, this could not always be a good thing. For example, extended magazines is almost always beneficial unless the magazine must be reloaded on a shell by shell basis (ex: Cowboy repeater), forcing a longer reload, or a scope on the other hand has pros and cons, as it may increase maximum range, but makes it harder to use in close combat situations.

The addition of a mod to a weapon will permanently increase the overall monetary value of that weapon by the value of the mod, even if the weapon is completely broken at the time it is applied.

Usage
Weapon mods are attached via the Pip-Boy. To attach a weapon mod you must have both the weapon it is intended for and the mod in your inventory. Highlight the weapon you want to mod within the items section of your Pip-Boy, then select the modding menu via the button as indicated in the top right of the info panel. After you have selected a mod, the weapon name will have a + after it to show it has been improved. Once a weapon mod has been installed it cannot be removed!

Pistols

 * ¹ The mod can also be attached to either the Laser RCW or Recharger pistol with full effects.

Rifles

 * ¹ The mod can also be attached to either the Laser RCW or Recharger pistol with full effects.

Vendors
There are currently 10 vendors who offer weapon mods in the Mojave Desert, with additional vendors included within DLC areas. Inventory of mods changes with stock rotation every 72 hours game time.

1 After completion of Honest Hearts

Achievements
Attaching 20 mods to weapons grants the Mod Machine achievement/trophy.