Griffin Armaments 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 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 concentric suppressor grade threads provide an optimized thread interface for your silencer regardless of brand or model. M4 feed ramps and chambers are polished prior to nitriding for smooth reliable feeding, chambering, and extraction. Gas ports were carefully engineered to provide reliable function suppressed or unsuppressed with 5.56 NATO ammunition. M4A1 contour diameter between the chamber and gas block for minimal POI shift with suppressors and maximum accuracy and durability while maintaining field weight. 416R SS barrel manufactured from stress relieved certified barrel steel. Black nitrided for extended service life. Magnetic particle inspected. Button rifled. 223 Wylde chamber for optimal accuracy. Mid length gas for softer recoil characteristics. .750 inch gas block journal. M4A1 diameter for reduced POI shift / Increased Accuracy and Durability. Air gauged and hand lapped bore. Polished chamber and M4 feed ramps. TRU-BORE suppressor quality threads. Suppressor optimized gas ports. .625 inch , 1/2x28 industry standard threads. Griffin Low-Pro RIGID rails are a slim, low profile, ultralight, yet rigid M-LOK rail. Design objectives of this rail were to provide a slim, ultralight rail, with increased rigidity over similar weight and profile current market options. A custom extrusion provides significant internal reinforcement to reduce flex when gripping the carbine firmly in the shoulder pocket. Understanding the importance of low light targeting, these reinforcements provide rigid mounting for IR laser aimers ensuring minimal POI shift when torque is applied to fore-grip type accessories, or when loads are applied to the rail (shooting off barricades, loaded bipods etc). Internal testing showed 70% less POI shift with laser aimers versus competing slim rails. Carefully positioned slots in the rail allow for rapid cooling when running the carbine fast while maintaining rigidity. The Low-Pro RIGID rail features a lightweight 7075 T6 Aluminum barrel nut, which can be installed with any car stock wrench or by using the included Griffin Armorers Wrench. Included shims allow barrel nut timing and keep the rear section of the rail low profile and ergonomically pleasing for relaxed posture carry at static positions. In the same way, flat socket head cap screws are used in keeping with the sleek design objectives. M-LOK slots at the 3, 6, and 9 o'clock positions allow for mounting of your favorite M-LOK accessories. Length: 13.5 inches. Height 2 inches. Internal Diameter 1.35 inches (measured from inside of reinforcement spars). Width: 1.54 inches. Weight: 9.2 ounces. Weight Installed: 10.52 ounces. Specifications for Griffin Armament MK1 Patrol 5.56mm 14.5 inch Upper: Manufacturer: Griffin Armament Caliber: 5.56x45mm NATO Barrel Length: 14.5 in Muzzle Device: Griffin Silencer Mount Muzzle Device Finish: Anodized Color: Black Model: Patrol Gun Rail: M-LOK Gun Model: AR-15 Magazine Included: No Length: 14.5 in Package Contents: Griffin Armament MK1 Patrol 5.56mm 14.5 inch Upper What firearm models does the manufacturer say Griffin Armament MK1 Patrol 5.56mm 14.5 inch Upper is compatible with? The manufacturer says that Griffin Armament MK1 Patrol 5.56mm 14.5 inch Upper is compatible with AR-15