Hartsfield's blades are Japanese-influenced, each one was hand ground from various kinds of tool steel, including 01 and A2. They are differentially edge hardened to RC 60-61 on the Rockwell scale with the remainder of the blade hardened to 58-59 on the scale to allow for flex during the cut. This strength protected the life of a Special Forces advisor in El Salvador when a Hartsfield knife stopped a 7.62×51mm NATO rifle bullet fired by a guerrilla. Hartsfield's blades were zero-ground, meaning there is no secondary bevel edge on the knife which allows the knife to cut without parasitic drag.