Weapon mod

, while they have appeared in one way or another in Fallout, Fallout 2, and Fallout 3, appear as an active feature for the first time in Fallout: New Vegas.

Fallout
Only Smitty can perform modifications in Fallout, and the only weapon he can upgrade is the Winchester P94 into a Turbo Plasma Rifle.

Fallout 2
In Fallout 2, you can pay a mechanic to upgrade certain weapons for you.

List of available weapons

 * Desert Eagle .44 → Desert Eagle (Exp. Mag.)
 * .44 Magnum Revolver → .44 Magnum (Speed Load)
 * AK-112 Assault Rifle → Assault Rifle (Exp. Mag.)
 * Colt Rangemaster Hunting Rifle → Scoped Hunting Rifle
 * FN FAL → FN FAL (Night Sight)
 * Wattz 1000 Laser Pistol → Magneto-Laser Pistol
 * Wattz 2000 Laser Rifle → Laser Rifle (Ext. Cap.)
 * Glock 86 Plasma Pistol → Plasma Pistol (Ext. Cap.)
 * Winchester P94 Plasma Rifle → Turbo Plasma Rifle
 * Flamer → Improved Flamer
 * Big Frigger Power Fist → Mega Power Fist
 * Cattle Prod → Super Cattle Prod

List of merchants

 * Valerie, in Vault City
 * Skeeter, in Gecko
 * Eldridge, in New Reno
 * Algernon, in the basement of New Reno Arms. Works for free.

Fallout 3
The only upgrades seen in Fallout 3 are scopes, the occasional use of silencers, and the use of expanded magazines. Other differences between unique weapons do not technically count as weapon mods, seeing as there is only one of each unique weapon. There is no way to get any weapon upgraded.

Fallout: New Vegas
It is still uncertain whether you can find parts and mod your weapons by yourself, or if the only way to mod weapons is through NPC's.

There are many types of mods, and early information states that there are three for each weapon type. Scopes, expanded magazines, even a mod to reduce the spooling time on the Minigun are all present. You can add three mods onto a weapon at once. Once you put a mod onto a weapon, it's permanent. So make sure you really do want that mod before you buy it.

List of merchants
Chet, in Goodsprings

Appearances
appear in Fallout, Fallout 2, Fallout 3, and Fallout: New Vegas.