You may need Chemist (1, 2 etc), Gunsmith (1, 2, etc), or other perks to unlock the ability to add a Weapon Mod. Some weapon mods can only be crafted / installed with certain perks.

Scrap weapons with mods to learn the Weapon mod pattern.

NOTE: Each weapon mod is attached to a particular weapon - so you can’t remove a mode from one weapon, then place it on another.

One of these methods requires a crafting table, and the other just gives you the option to apply a mod directly to a weapon. Mods are weapon customization features you can unlock to enhance your weapons at the crafting table. How To Unlock Weapon Mods | Gun Customization Guide

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Fallout 76: Where To Find The ‘Rose’s Syringer’ Gun | Unique Weapon Guide.Fallout 76: How To Join The Enclave & Get The Laser Minigun | Faction Guide.Fallout 76: Power Armor Frames, Core Farming & Unique Set Locations Guide.Fallout 76: How To PVP Without Becoming Wanted | Bonus PVP Modes Guide.Fallout 76: 10 Things You Should Do First | Power Armor Location & More | Beginner’s Guide.There are literally dozens of mods to unlock for some weapons, so I recommend grabbing everything you can, and scrapping everything you can. You’ll earn lots of different mods by scrapping Pipe guns, but you can also get mods for Syringer Rifles that drain enemy strength, longer barrels for your shotgun to improve accuracy, or special receiver that change the firing rate on assault rifles. To unlock mods, you just need to scrap weapons with the mods you want. You’ll have to grab each mod separately, and you’ll do that naturally as you play - if you’re scrapping weapons. Because of the perk card system, there’s no straightforward way to simply unlock new mod options when you’re customizing weapons. After the defeat of the Enclave in the Core Region, the Plasma Carbine was phased out in favor of the Urban Plasma. While it ultimately lost out to the RobCo/REPCONN Urban Plasma Rifle, it remained popular with Enclave personnel when they were able to get their hands on one. These extra pieces are called “mods” - and installing them has changed slightly. The Winchester P95 Plasma Carbine was developed by Winchester as a replacement for the bulky, demanding Plasma Caster. Weapon customization returns in Fallout 76, and you can mod your weapons up with a variety of powerful configurations.

Fallout 2 plasma rifle