Weapon Modification or weapon mods is a feature in the upcoming game Fallout: New Vegas. Although it somewhat appeared in Fallout 2 and Fallout 3, in Fallout: New Vegas it is expanded and is considered a legitimate feature.
 Fallout 2
In Fallout 2 the player can pay shop-owners/mechanics to fix and upgrade weapons. These changes were usually minor though, such as the AK-112 Assault Rifle → Assault Rifle (Expanded Magazine).
 Fallout 3
In Fallout 3 weapon modification isn't directly talked about or mentioned, but the player can find silenced and scoped weapons. The player could not attach these mods to the weapon themselves though.
 Fallout: New Vegas
In 'Fallout: New Vegas the player can pay a merchant to put several mods on his/her weapon. Once you put a mod on a certain weapon, it is permanent and will stay on the weapon forever. There are scopes, silencers, stocks, extended barrels, and other things you can add to your gun, although you can only have three at a time.
The only known weapon modifier is Ringo at this time.
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