1933 Model 21 catalog included the 115,000 psi tensile strength figure for "Winchester Proof Steel", and this



7.5 long tons = 16,800 psi + 10 - 14% for modern transducer number. 16,800 + 1,680 = 18,480 psi. Modern SAAMI proof load standard is about 19,000.
5 long tons = 11,200 psi + 10 - 14%

SGJ has a good point regarding the low concentration of nickel content - it was no doubt on purpose.
4340 has .7 - .9% Cr and 1.65 - 2% Ni
4330 .4 - .6% Cr and 1.0 - 1.5% Ni

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