Not knowing much about the gun, apart from reading a magazine article raving about ST barrels a while back, had a Google;
PRODUCT DESCRIPTION
The new FX Airguns Superior Smooth Twist X Barrel Liner. Produced in the FX patented way; by pressing the rifling into the barrel from the outside. But now with a higher level of precision, lower tolerances and other tweaks, resulting in a superior liner. There are two types of liners being released for most calibres; a standard Superior STX Liner and a Superior STX Heavy Liner. The standard FX Superior STX Liner can shoot a standard weight pellet and Hybrid Slugs, maximising the accuracy of the FX Hybrid Slug. The FX Superior STX Heavy Liner will be replacing the STX Slug A liners in the coming months. Heavy denotes that this liner has a different twist rate better suited for heavier pellets and heavier slugs (as opposed to lighter weight Hybrid Slugs).
Superior
Standard Liner used on all production models and barrel kits. Created to shoot standard weight pellets and Hybrid Slugs. Also Maximises the accuracy of the FX Hybrid Slug. Lighter Weight slugs from some manufacturers may aklso shoot well.
Superior Heavy
Only sold as separate accessory (not as barrel kit). Has a different twist rate better suited for heavier pellets and heavier slugs (as oppposed to lighter Hybrid Slugs and some manufacturers lightweight slugs
Webley Mk3 x2, Falcon & Junior rifles, HW35x2, AirSporter x2, Gold Star, Meteors x2, Diana 25. SMK B19, Webley Senior, Premier, Hurricane x 2, Tempest, Dan Wesson 8", Crosman 3576, Legends PO8.