Allen Engineering AEM4 (FDE, Ops 16th Model). 5.56mm. Same baffle section as the AEM5 with a shorter mounting length — interchangeable with AEM2 and AEM3 mounts. Fits barrels as short as 11.5" with a low-profile gas block. Length: 8.4". Weight: 20oz. Minimum barrel clearance: 2.7". Mount length: 2.05". Compatible with standard AE/OPS brakes and collars. 7-10 turns for best fitment. Note: If this product is reflected as in-stock it means it's either physically in-stock or allocated and available (serial number is assigned) at my distributor