FX has upgraded their STX barrel liners by creating the Superior Smooth Twist X (Superior STX) liner system. It has the same game changing exterior pressed rifling, but they've enhanced the production methods and tightened tolerances to get even more precision. This Superior STX liner shoots slugs and pellets with better accuracy than previous STX liners. You get both convenience and power with this 500mm model. If you're using the FX Hybrid Slugs, you'll get the best accuracy using these new Superior liners. It's a quick upgrade for your FX airgun, so buy a new Superior STX barrel liner to swap in now.