The AAC BlackOut Flash Hider for 51T mounts is an extremely efficient design that effectively eliminates muzzle flash, even on short barrels. These are a pingless design once torqued 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. 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 the AAC 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. Note: Rocksett Included