Steve4102 asks:
Quote:
Has anyone ever done pressure testing to determine at what pressures the most looked for pressures signs actually occur?
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Well, sort of.
Some years ago I was shooting Lake City Arsenal M60 proof loads in 7.62 NATO chambered M1 Garands. The load was an LC-65 case weighing 180 grains, FA-956 primer, 41 grains of IMR4475 (very similar to IMR3031, but much cleaner burning) and a 173-gr. FMJBT bullet. In Lake City's proof test barrels, it produced 67,500 CUP (about 84,000 PSI). After firing a dozen of them, the cases didn't show any imprint of the holes in the bolt face for ejector and extractors to fit in. That same set of components in a new Federal brass .308 Win. case of 170 grains weight fired in the same chamber had very visible imprints of the bolt face holes in their heads. That same LC-65 case, powder charge and primer normally had a 148-gr. FMJBT bullet in it as the standard M80 service round producing 50,000 CUP (about 57,900 PSI)
Which tells me the Federal case brass was softer than the LC brass. And harder brass will have less peak pressure signs than softer brass.
Same thing with primers; those with soft or thin extrude more around the firing pin into its hole in the bolt face with the same amount of pressure.
In my opinion, estimating peak pressure from visible signs, or measurements, on fired cases is not going to be very accurate.
A bolt that's hard to lift may have its face way out of square. Cases fired in chambers tend to have their heads flattened out closely matching the bolt face angle. Sizing those cases doesn't square up their heads. Firing a case from such a condition will sometimes bind the bolt when the high points of the case head and bolt face align with each other. Even with normal safe peak pressures with fired cases partially neck sized with a full length sizing die, this happens and is often believed to be caused by peak pressures bing too high.