Regarding 4340, the Turkish BRG-9 Elite handgun is advertised as having the barrel and slide made from this grade of steel. Don't know if they're unique in this or if there are other handguns with similar specs.
"They then subject the slide and barrel to a double heat-*treatment process. That is done to produce a harder surface while maintaining a tough core or interior, which keeps hard parts from being perhaps a bit brittle. In effect, you get the surface hardness of a 4350, and they get the machinability of the 4340 alloy. This is not a new idea, it has been done for over a century in manufacturing, but it does take extra time, steps and equipment, so hats off to Burgu for going those extra steps for our benefit."
https://www.firearmsnews.com/editori...9-elite/467664
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