The AAC BlackOut Flash Hider for 51T mounts is an extremely efficient design that effectively eliminates muzzle flash, even on short barrels. These are the new pingless design once torque to the correct spec. These will allow you to have most any 13.7/13.9 barrels to be 16a ^2 OAL. Please double check before pinned and welding them for total length. All NFA rules apply. The open-end, 3-prong design of the BlackOut is inherently stronger than 4-prong units and is not subject to the wear and erosion commonly encountered with closed-end designs. All of our BlackOut flash hiders are machined from high strength aerospace steel alloy and coated in a hard, durable, corrosion resistant SCARmor finish. These will be machined with a pre-drilled pilot hole for easy installation of a pinned and weld.