This is one of many historical documents available on DTIC. I found the following interesting:
"During the test, it was noted that the phosphate-coated test bolts averaged more than 10,000 rounds each and only one failed. The remainder were removed due to the conditions of the test bolt carriers . This increase in bolt life is attributed to use of phosphate coating instead of hard chromium plate, thereby avoiding a condition wherein ·the hard chromium can be fractured easily leading to rapid propagation of the crack through the hard case and· soft core of the bolt."
So, some 53 years later, is this still a valid conclusion - that Parkerizing the bolt led to increased life over chrome plating?
https://apps.dtic.mil/dtic/tr/fulltext/u2/483309.pdf
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