From the HBI site
"GUNSMITH INSTALLATION IS HIGHLY RECOMMENDED. The Scorpion Evo family of firearms span several generations (Gen1, Gen2, ect?) and the trunnions to which the barrels mate are not clocked/timed. This means that each barrel must be paired exclusively to the trunnion/upper receiver in which it will be used. As such, our barrels DO NOT have an extractor notch milled into the chamber. This notch must be machined and matched directly to the trunnion in YOUR Scorpion Evo. Please see our video below for more info. Please note that this barrel does NOT work with factory handguards (pistol or carbine)."