Some give better hip-fire accuracy, while others improve accuracy when aiming down the sights.

Barrel - This affects range, accuracy, and recoil.

Some weapons can also change the ammo type in this category.

This is how you change a weapon between normal or automatic.

Receiver - This affects the weight, damage, and rate of fire.

Classic Guns (Non-energy weapons like pistols or rifles) I'll list the types of mods you'll see in each gun type. Most weapons have multiple types of mods in common, but each category has mods unique to that type. Note: If you use an item during crafting instead of breaking it down for materials, you lose any extra materials that the item has that are not used in the recipe. To do this, drop what you want to scrap, open the workshop menu, and highlight the item to scrap it. I suggest grabbing nearly everything you come across while traveling because you can break it down into materials at a workshop. There are others, but it is best to check each weapon. Below are examples of other materials you might need. Every weapon mod needs Adhesive, so stock up. The materials needed depends on the gun, type, and mod. Melee and Heavy weapons are in Strength, Rifles are in Perception, and Automatic or Pistols are in Agility. You will also want 2 or more weapon damage perks, but these aren't required to craft anything.

Science (Int 6) - This lets you craft high-tech mods.

Blacksmith (Str 4) - This lets you craft melee weapon mods.

Gun Nut (Int 3) - This lets you craft mods for guns.

If you plan on taking advantage of the weapon mod crafting system, there are multiple perks you need to keep an eye on. Luckily, there are many throughout the world and you can add them to your settlements. You need a weapon workbench to craft any mods for weapons.

Advanced Weapon Mod Crafting - Each weapon category and the types of mods you can make for each.

Weapon Mod Crafting Basics - How to craft mods, perks needed, and examples of materials.

Don't forget to check the Fallout 4 Beginner Tips and Tricks for more guides! This guide will go over everything about crafting weapon mods including:

I'm going to describe the mods available for each weapon category and what you need to make them.

Fallout 76 weapon mods list