(J.Kuhnhausen's Mausers shop manual) Case hardening of early mauser receivers as he tested ran approx .002"-.008". A hardness of .002-.005 is considered generally thin. M98's test equivalently to 1035 and 1036 steels. The 1035's generally gave rockwell C33-35 exterior hardness. FN's factory hardness test indentations were at the side of the recoil lug and on the flat bottom of the receiver behind the feed ramp.
Mauser98 case hardening thicknesses post-1936 vary but avg .008-.010" and up to .012-.018" were occassionally found. Ideal case thickness is .010-.015" w/ typical exteriors to a hardness of C35 to C40, and internal hardnesses of C22 to C30.
If a sharp awl or knife easily penetrates or scrapes the surface then the hardening should be considered too thin. Hardening is critical to prevent action lug area setback, w/ acceptable lug surface wear seating of up to .002". Interestingly, bolt lug C-scale readings run from 50-55 hardness generally, which provides a bolt tensile strength that can exceed 100,000 lbs.
His suggestions may shed light that receivers once hardened could be apparantly lightly buffed or lightly struck/etched with a 32percent HCL acid solution and then rust or oxide blued.
