Proof Research 257 Caliber Non-Threaded Rifle Barrel Sendero 22 in 7.5 Twist Pxt 3.2 Weight Carbon. 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 257 Caliber Non-Threaded Rifle Barrel: Manufacturer: Proof Research Projectile Caliber: .257 Caliber Threads per Inch: Non Threaded Barrel Twist Rate: 1-7.5 Barrel Material: Stainless Steel Flutes: No Gun Make: Proof Research Package Contents: Proof Research 257 Caliber Non-Threaded Rifle Barrel Proof Research 157510: 257 Caliber Non-Threaded Rifle Barrel Sendero 22 in 7.5 Twist Pxt 3.2 Weight Carbon