In the later models he used Krupp steel for the barrels, which I believe are either 4130 or 4140 steel. He used VGM steel for the receivers which I have heard was 4340 steel. 41XX steel is chrome-moly; 43XX is chrome-moly-nickel. The last two digits in the 4130 or 4140, or 4330 or 4340 refer to the carbon content (.30% or .40%).

Modern barrels tend today towards 4140 steel for the harder-wear resistance characteristics over the 4130 steels.

Jim Haynes