Open beta for the 556 Long variant in the HubTN.4 series. HUB as well as direct thread (3/4x16) attachment options are included as usual. There are ~1.68" from the base to the lowest point of the blast baffle which accommodates the majority of QD muzzle devices if you go the HUB route. This design's little brother the 556k,k+ has already been demonstrated to work with 556 (at least with a 16" barrel and a moderate rate of fire per the thermal comparison video on reddit).