Weapon mod

Weapon mods have appeared in one way or another in Fallout, Fallout 2, and Fallout 3. They appear as an active feature for the first time in Fallout: New Vegas due to their popularity as a third party mod $$ New mods were introduced with the new crafting system in Fallout 4. Fallout 76 has a similar system of weapon modding except one needs to scrap weapons in order to learn the mods and it decreases max weapon durability.

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, one can pay a mechanic to upgrade certain weapons for them.

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

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 4
Weapon mods can be built from raw materials and attached to weapons at weapons workbenches. There are over 700 mods, including scopes, barrels, stocks and more. For guns the Gun Nut perk is required, while Science! is required to modify energy weapons and advanced modifications for guns and Blacksmith is required for most melee weapons mods.

Most pistols, rifles and shotguns offer the option to customize grips/stocks, receivers and sights, changing which perks affect the weapon.
 * Any weapon with an automatic receiver gets bonuses from the Commando perk.
 * Pistols with non-automatic receivers get bonuses from the Gunslinger perk.
 * Rifles and shotguns with non-automatic receivers get bonuses from the Rifleman perk.
 * The Sniper perk only works on non-automatic rifles with scopes which use the "hold breath" mechanic.

Grip mods create pistol weapons, while stock mods create rifle weapons. This allows for almost any ranged weapon to be customized to fit the player character's build.

Fallout 76
In Fallout 76, weapon mods are done similarly to Fallout 4 with the main differences being one has to learn the mod by scrapping weapons and it reduces max condition.

Appearances
Weapon mods appear in Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout 4, and Fallout 76.