To add some perspective to the data above...my experience suggests that many modern firearms that are made from 4140 are hardened and tempered to around the low to mid 30's HRC (hardness) giving an ultimate tensile strength from about 145,000 psi to 165,000 psi. Mild steel has an ultimate tensile strength of about 64,000 psi. Case hardening is in the order of .003-.007" and does not change the core strength. Yield strength numbers are lower for both.