Save big on this Aero Slick Side Assembled Upper + Toolcraft Nickel Boron 158 BCG Combo! Toolcraft Nickel Boron 158 MPI BCG:This Nickel Boron 158 BCG from Toolcraft features an MPI bolt and M16 carrier, and comes with a lifetime warranty. Built to military specifications, it stays true to the Toolcraft name of quality, durability and best-in-industry. The bolt is made from mil-spec carpenter 158 steel, shot peened and magnetic particle inspected, while its carrier is made from 8620 tool steel. All components are pre-polished before being plated by UCT and then polished again for a high shine.