It is currently untested whether or not this applies to the weapon noise from the weapon itself or from the weapon and the projectile.

Silence: No values used, this silences the weapon.

Increase Clip Size: Value #1 is a flat bonus to the weapon's clip size, or amount of shots that can be taken before reloading.

Increase Projectile Speed: Value #1 is the multiplier of the projectile speed.

Value #2 doesn't currently appear to do anything, but this requires further testing to ascertain whether that is the case. The weapon then ignores the next use of ammunition.

Regenerate Ammo (Shots): Value #1 is the number of shots before the regeneration effect procs.

If negative, it reduces the time taken to recharge.

Regenerate Ammo (Seconds): Value #1 is the number of seconds added to the weapon's recharge time.

Increase Max Condition: Value #1 is a flat bonus added to the weapon's maximum condition.

Increase Weapon Damage: Value #1 is a flat bonus to the weapon's damage.

Decrease Weight: Value #1 is a flat reduction to the weapon's weight.

Very few modification effects use the second value. Each mod can have two values specified- each does a different thing depending on the effect specified. A weapon can make use of up to 3 mods, each of which can be assigned one of the effects from the list below. Mods are assigned effects on the weapon's Mod Info tab. In addition, attached Weapon Mods are effectively destroyed and merged into the weapon itself- they do not run their scripts, and cannot be detected using the GetEquipped function. If the script contains OnAdd blocks, the game will crash when the mod is attached. However, they do not run their OnDrop or OnEquip blocks when they are attached to a weapon. Multiple different weapons can therefore use the same mod and grant different bonuses.

Note, Weapon Mods have no effect in and of themselves- the only data they have is text description. They can be selected as 'mods' on a weapon's Mod Info tab. Weapon mod items, known in the GECK as Item Mods, are new items in Fallout: New Vegas roughly similar in behaviour to Misc items.

Fallout 4 remove weapon mods