5.56x45mm NATO Federal BTHP 62 Grain 20 Round Box MK318 MOD-O USMC "SOST" Match Bullet T556TNB1. The military requested these be marked seconds to be sold to the civilian market.
This new SOST round from Federal Cartridge was engineered for the USMC as a supplemental replacement round to M855 green tip with more desirable terminal characteristics. NSN # 1305-01-573-2229. Designated as MK318 MOD-0 the round was designed as a barrier defeating round with superior penetration and better ballistic stability when shooting through glass, car doors and other barriers where other rounds perform poorly. It was engineered after the Marine Corps identified issues with the M855 round. This round utilizes a 62 grain open tip boat tail hollow point bullet with a lead core and reverse copper jacket creating the open tip.