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
 * Varmint Rifle → Night Scope

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 extended magazines. Other differences between unique weapons do not technically count as weapon mods, seeing as there is only one of each unique weapon.

Fallout: New Vegas
Any of New Vegas's weapons can be modified with several mods.

There are multiple weapon mods for every weapon, and a maximum of three weapon mods can be attached to any one weapon. Once any weapon mod is attached it is permanent, so be wise in what you want to attach. For example, while an extended magazine is almost always beneficial, a scope has pros and cons. It may increase maximum range, but it may also worsen your ability to aim closer to yourself, making it harder for close combat.

Weapon mods can be found at certain weapon merchants, including the Silver Rush and the Gun Runners in New Vegas.

Guns

 * Pistols
 * .357 magnum revolver
 * .357 magnum revolver long barrel
 * .357 HD cylinder
 * .44 magnum revolver (Fallout: New Vegas)
 * .44 revolver scope
 * .44 revolver heavy frame
 * 10mm pistol (Fallout: New Vegas)
 * 10mm pistol silencer
 * 10mm pistol extended mags
 * 10mm pistol laser sight
 * 9mm pistol (Fallout: New Vegas)
 * 9mm pistol extended mags
 * 9mm pistol scope
 * SMGs
 * 10mm SMG (Fallout: New Vegas)
 * 10mm smg extended mags
 * 10mm smg recoil comp
 * 9mm SMG (Fallout: New Vegas)
 * Drum Magazines
 * Light Bolt
 * Rifles
 * Hunting rifle (Fallout: New Vegas)
 * Hunting rifle extended mag
 * Hunting rifle scope
 * Hunting rifle custom action
 * Varmint rifle
 * Varmint rifle extended mags
 * Varmint rifle night scope
 * Varmint rifle silencer
 * Cowboy repeater
 * Custom Brass Plating
 * Maple Wood Grip and Stock
 * Cowboy repeater long tube
 * Sniper Rifle
 * Sniper Rifle Suppressor
 * Sniper Rifle Carbon Fiber Parts
 * Service rifle
 * Service Rifle Upgraded Springs
 * Service Rifle Forged Receiver
 * Assault carbine
 * Assault Carbine Extended Magazines
 * Shotguns
 * Hunting Shotgun
 * Hunting shotgun longtube
 * Hunting Shotgun Choke
 * Heavy
 * Minigun (Fallout: New Vegas)
 * Minigun Damped Subframe
 * Minigun High-Speed Motor

Energy Weapons

 * Rifles
 * Laser Rifle
 * Laser Rifle Focus Optics
 * Laser Rifle Beam Splitter
 * Laser Rifle Scope
 * Plasma Rifle
 * Plasma rifle mag accelerator
 * Laser RCW
 * Laser RCW Recycler
 * Heavy
 * Plasma caster
 * Plasma Caster HS Electrode
 * Gatling Laser
 * Gat. Laser CF Frame

Explosives

 * Grenade
 * Grenade Rifle
 * Grenade Rifle Long Barrel - Increases range (+100%)]

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