Weapon parts are the resource used to upgrade the level of your weapon and in doing so it's damage output. Each weapon has three levels that adds +1Dmg and +1Crit DMG per level with the first level being the weapon's base damage. The second level costs 30 parts and the third costs 60 parts meaning that you will need 90 parts to fully upgrade a weapon, however it is higher suggested that you upgrade your stealth weapons first as you will always use them meaning they're not likely to become obsolete.
If you have a weapon you no longer want or need you can break it up at the Fix Pit and gain weapon parts for it, the following is a list of weapons and their worth.
|Elysium - LR678||29|
|Mimir needle pistol||27|
|Mimir - PC98||27|