An FCU (Fire Control Unit) is needed for all Polarstar rifles with the acception of the Polarstar Kythera. If buying this with a Polarstar rifle from airsoftUS it will come pre tuned for that specific rifle. If your buying this as a solo product and you would like it tuned for your rifle please fill out the contact form or leave a comment giving the following details, the more specific the better
Which Polarstar Engine - Ex. Polarstar Jack
Hopup unit ------------- Ex. Maxx me pro M4A
Bucking ---------------- Ex. Maple leaf macron 70 degree
