Proof Research 308 Caliber Non-Threaded Rifle Barrel Competition 26 in 10 Twist Pxt 7.13 Weight Silver. PXT - PROOF Exponential Twist - is a complete rethinking of conventional rifling geometry and twist progression engineered to reduce engraving force minimize throat erosion and create a smoother transition as the projectile engages the rifling. Specifications for Proof Research 308 Caliber Non-Threaded Rifle Barrel: Manufacturer: Proof Research Color: Silver Projectile Caliber: .308 Caliber Barrel Length: 26 in Barrel Profile: Competition Threads per Inch: Non Threaded Barrel Twist Rate: 1-10 Barrel Material: Stainless Steel Flutes: No Gun Make: Proof Research Weight: 7.13 lb Package Contents: Proof Research 308 Caliber Non-Threaded Rifle Barrel Proof Research 155127: 308 Caliber Non-Threaded Rifle Barrel Competition 26 in 10 Twist Pxt 7.13 Weight Silver