IIRC, the name for that finish is "French Grey".
While, on some firearms, French Grey can simply be coated bare/polished steel, AFAIK it's usually a plating process.
Plated French Grey is a soft looking, hard gray-metal finish which is superior to stainless steel in its corrosion resistance, wear ability, and operational "slickness".
The exterior metal surface is prepped by blasting with a blend of very fine aluminum oxide and glass bead media - which is what actually what causes the deep "French Gray" look after plating is complete.
The final French Gray surface is a two-part metal plating process that bonds to steel at the molecular level, resulting in a hermetic seal that protects against corrosion while beautifying the firearm it’s applied to.
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