If i have a modded weapon and sell it to a vendor because its dead weight and i don't really use it, will that still count towards the achievement? Or do i have to hold on to the weapon the whole time.
Once you have made the mod it's in the game forever, tallied so to speak. You don't have to keep the weapon after it's modded. I modded all the lasers and then sold them as you did. Still got the chievo.