DO NOT ORDER THIS PART UNLESS YOU ALREADY HAVE ONE OF OUR FRAME MOUNTED LOWERS. The Underworld Arms Basilisk Upper is intended for customers who already own one of our frame mounted compensators and want to swap to this new style upper. This compensator upper features two large top ports and four smaller angled side ports.  On most builds recoil spring tuning WILL be required. The stock recoil spring is 12lbs, with this compensator a spring in the 8-10lbs range will usually be necessary for proper function unless high power loads are being used.    Specifications Material: 7075-T6  Weight: 1.2oz Length Overall: 1.5"