Gun barrel steel is heat-treated through to about 28 on RC scale with no carbon added. .22 and old low pressure round barrels were made of lower grade steels with no heat-treating and some may still be. Point is, you have to introduce carbon to "Soft steels" to harden them. High power barrel steel could heat-treat itself just from welding on it. Welding usually anneals gun barrels, but it could go the other way. Under the right conditions, barrel steel can be made so hard that a carbide endmill will barely put a dent in it.
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