Proof Research AR-15 300 Blackout Threaded AR Barrel Standard Standard 12.5 in Gas Length Pistol 7 Twist Pxt 2.8 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 AR-15 300 Blackout Threaded AR Barrel: Manufacturer: Proof Research Caliber: .300 AAC Blackout Barrel Profile: Standard Gun Make: AR Platform Gun Model: AR-15 Gun Type: Rifle Package Contents: Proof Research AR-15 300Blk Threaded AR Barrel What firearm models does the manufacturer say Proof Research AR-15 300 Blackout Threaded AR Barrel is compatible with? The manufacturer suggests that Proof Research AR-15 300 Blackout Threaded AR Barrel is compatible with AR-15Proof Research 157176: AR-15 300 Blackout Threaded AR Barrel Standard Standard 12.5 in Gas Length Pistol 7 Twist Pxt 2.8 Weight Silver