Pressure Signs

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Pressure Signs

Postby Siringo » Mon Jul 13, 2009 1:06 pm

During my limited experiments with heavier than factory bullets (300 grs to 400 grs) I have noticed extractor/ejector marks more often than with the lighter bullets. Even though the pressure signs on the case (primers and case head expansion) will nill. This leads me to ponder this:

The typical 450 case with primer weights 170 grs. With my 240 gr. load is use 36.2 grs of LG. Total weight of case and powder is 206.2 grs. The ratio of the case/powder weight to bullet weight is 0.86 (206.2 grs divided by 240 grs). With my 400 gr. jacketed load, is use 27 grs of LG. The ratio on that round is 0.49. My factory equivelant is 0.82.

When any cartridge is fired, the following takes place.

1. Firing pin drives round further into chamber (takes up headspace variation)
2. Primer fires and backs out of case slightly due do variations in headspace tolerances and internal pressure.
3. As pressure builds, case moves to the rear the tolerance variation - as bullet remains in a fixed position related to the bore (case is moving, not bullet). This action also seats the primer flush with the back of the case.
4. Pressure mounts as cases seals in chamber.
5. As case has stopped in rearward travel, pressure continues to mount and the bullet begins to move forward.

My thoughts here (and this relates only to my AR) is the heavy bullet/powder combination will move rearward faster and with more energy against the bolt face than a lighter bullet/powder combination. It is possible that the marks I see on the back of my cases related to ejector swipes and such, are really not signs of high pressure. I had shot some 400 gr. lead bullets with charges of 5744 and velocities of 1300 fps and still had these marks.

What also brings me closer to this is the letter stamping on the cases used for the heavier chargers is beaten smaller than the other cases. Are the cases essentially being peened? Comments? I am I all wet?

Certainly there is a myriad of other factors that may contribute. If the case length was exactly the same as the chamber, this may not happen at all. Also what leads to me suspect this even more, is that I inadvertanly trimmed some cases .005" inches shorter that I wanted to. These cases show the markings even more.
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Re: Pressure Signs

Postby pitted bore » Tue Jul 14, 2009 8:59 pm

Siringo wrote:My thoughts here (and this relates only to my AR) is the heavy bullet/powder combination will move rearward faster and with more energy against the bolt face than a lighter bullet/powder combination. It is possible that the marks I see on the back of my cases related to ejector swipes and such, are really not signs of high pressure. I had shot some 400 gr. lead bullets with charges of 5744 and velocities of 1300 fps and still had these marks.

What also brings me closer to this is the letter stamping on the cases used for the heavier chargers is beaten smaller than the other cases. Are the cases essentially being peened? Comments? I am I all wet?

Certainly there is a myriad of other factors that may contribute. If the case length was exactly the same as the chamber, this may not happen at all. Also what leads to me suspect this even more, is that I inadvertanly trimmed some cases .005" inches shorter that I wanted to. These cases show the markings even more.


Siringo-
I've some minor quibbles with your sequence, but they aren't pertinent to your main point, which is intriguing.

Assuming equal peak pressures with the light and heavy bullet cases you describe, I don't think rearward speed of motion of the case is going to make that much difference in pressure indicators. I suspect that any difference in case head deformation (pressure indicator) caused by a difference of a few feet per second (I admit not to know how fast the rearward motion is) will be swamped by the application of about 20 tons of pressure per square inch.

If you can be sure that the peak pressures with the light and heavy bullets are equal, then your observations and thinking about what caused the difference in "pressure signs" will be worth careful study.

Do the particular cases that show the peening of the headstamps also show expansion of the diameter of the case heads?

Thanks for bringing this up.
--Bob
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Re: Pressure Signs

Postby BD1 » Wed Jul 15, 2009 7:40 am

In the AR action, you also have the timing of the pressure curve to consider. The use of slower powders can mean that there is still enough pressure in the barrel to keep the case expanded tight to the chamber walls when the bullet passes the gas port. This means that the bolt is starting to rotate and unlock while the case is resisting this movement. I think this is the logic behind Wildcatter's relocating the gas port farther down the barrel.
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