Proof Research 224 Caliber Non-Threaded Rifle Barrel Competition 26 in 8 Twist Pxt 7.9 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 224 Caliber Non-Threaded Rifle Barrel: Manufacturer: Proof Research Color: Silver Projectile Caliber: .224 Caliber Barrel Length: 26 in Barrel Profile: Competition Threads per Inch: Non Threaded Barrel Material: Stainless Steel Flutes: No Gun Make: Proof Research Package Contents: Proof Research 224 Caliber Non-Threaded Rifle Barrel Proof Research 155103: 224 Caliber Non-Threaded Rifle Barrel Competition 26 in 8 Twist Pxt 7.9 Weight Silver