Griffin Armament's MK1 Upper offers premium features and reliability to each user and allows end users to configure their upper to fit their needs. Standard and available features: The Griffin HEDP(TM) Barrels provide High Endurance Dual Purpose functionality- offering high durability and precision accuracy. Nitrided 416R stainless steel provides the highest possible hardness and longest throat life available with match grade bore dimensions and bore finish. HEDP barrel bores are hand lapped, ensuring batch to batch consistency and accuracy. TRUE-BORE(TM) concentric suppressor grade threads provide an optimized thread interface for your silencer regardless of brand or model Mfg: Griffin Armament