With strength in mind, Toolcraft made their Bolts from heavy duty 9310 Alloy Steel and their Bolt Carriers from 8620 case-hardening aircraft grade steel. 9310 steel has a high core strength rating, high fatigue strength, and a high hardenability rating overall. Normally, 9310 steel is used for gears and crankshafts in massive trucks and large aircraft, so its easy to see why it would make an ideal Bolt for an AR. Why 9310 instead of Mil-Spec Carpenter 158 steel?