These have been around as long as we have...Not new. They aid in extraction.
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How do you think the flutes were produced then if they weren't broached or forged?
If the stub that you cast is dated 1959 I guess that it would be in the era of "spark erosion technology" but I don't know how good the process would be in this application, i.e. a gun chamber.
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That is why I wondered if they broached the flutes individually but as you suggest it is deep.
The sharp edges appear to be cut , like broaching but the end angles into the chamber would be almost impossibly cut at the sharp angle.
If I had a spare barrel to keep I would prefer to section it and have a closer look.
It might be an insert not visible externally.
Some M60 barrels used a hardened insert for wear.
Maybe its something similar , I have no idea.
M60 Stellite Liner: A Closer Look - YouTube
When I saw that fluted chamber I couldn't help thinking of all the unburned propelland and carbon that would end up in there. Just remember all that diamond hard crusted-on carbon fouling that you used to spend hours cleaning off of the gas parts while sat on the ranges. That same crap will be encrusting the fluted chamber. Just my view of course because it's got to go somewhere and gas ALWAYS takes the line of least resistance
Our early GPMG's had stellite lined barrels for the first 6" or so. The problem was that the stellite was good, but it just eat the barrel FORWARD of the stellite liner!
The Soviet SVT 40 used a fluted chamber (do not believe full length but still fluted) not all used the fluted chamber but a large number did use it.