Brian can correct me if I am wrong on this,but the trigger guard and other parts were nitre blued for cost and because they are not of as high carbon steel as the barrels and don't take rust blue as well.Also,the main purpose of case hardening is to provide a hard outer shell around the action but still allow the action to flex on recoil.If the action is overhardened it becomes brittle and subject to cracking.The colors are just a byproduct of the process.
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