A thin wash of copper is often an indication that a gun has been cold blued. Sometimes, a cold bluing job can be detected by a copper smell. A dilute acid will remove it, but you have to be careful that the acid does not etch the steel, unless you desire a matte finish. You could also look into reverse electroplating with the copper plated frame as the anode, and have it thus removed and plated out onto the cathode.

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