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Reloading Techniques For Reloading


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Pressure Signs

 
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Old 12-05-2013, 05:03 AM
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Join Date: Jul 2012
Location: Washington State
Posts: 462
Re: Pressure Signs

Please allow me to interject,

P.O. Ackley said it best, "The absolute pressure of a cartridge is almost solely dependent on the brass case itself. Once the mechanical limit of the brass is exceeded the pressure, no matter what it is, is excessive."

When you see the signs on the brass case itself, the pressure is too much because the brass has exceeded its elastic limit. Doesn't matter what the actual pressure is. Case closed.

Woods: Head space (too much of it) causes flattening. Very true. So does too much pressure. Which one are you looking at? Tough call. I have a Pressure Trace II on 5 guns. When it says the loads are hot the primers are flattened and mushroomed. The primer shank inside the pocket stays at 0.210" but the crown of the mushroom will be about 0.212" for large rifle primers. Cratering can be caused by high pressure or a lousy fit of the firing pin. Look at the bolt face with the pin protruding. Use a magnifying glass. Can you see a gap? Get it fixed! When the primers start cratering trouble is very near...
Another sure sign on the primers is how defined the machine marks of the bolt face are transferred to the primer surface. Magnifying glass again. Use an unfired case and primer to compare. Sometimes the priming punch will lightly mark the primer face too. If the primer looks like a lithograph of the bolt face the pressure is very high, but probably safe. +mushroom +mild cratering and the bolt face markings are signs its too much. I should try to post some micrographs (very close photos) of the primers.

varmintH8R: There is a strange phenomenon seen in pressure traces after the bullet leaves the barrel where the pressure rises again. It has been noted to reach as high as 80kpsi in loads that are safe at 62kpsi at the chamber while the bullet is accelerating down the bore. Why would the pressure rise after the bullet unplugs? Don't know. One load in my son's 300WinMag- Barne's Max with RL19 and the 150gr TTSX- shows a pressure rise of about 8kpsi after the bullet unplugs. Still very safe at about 33kpsi at that point, but why? The strain gauge is centered 1.575" forward of the bolt face so the chamber is seeing this pressure- it's very real.

BrentM: You're astute having spotted the brass problem. Doesn't really matter why except to correct it. That load is a problem so don't shoot any more. Bullet pulling time! My hunch is the brass is screwed up.

But it's still a hunch...
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