Salvaging Ammo Parts and Age

[Parashooter]

Shoulder setback from primer force in rimless bottleneck cases is well-documented, though most often in conjunction with low-power reduced loads which never reach enough pressure to force the case head back against the bolt face and expand the shoulder forward again. The reason it occurs is that the priming compound is a very strong explosive, partially confined by the primer pocket, causing the primer cup to act as a piston driving itself back and the the case forward, deeper into the chamber.

For more detail see shoulder set-back

The following illustrates the sequence of events on firing - but shows a rimmed cartridge, so we have to imagine what happens with rimless when the primer detonates -

Information

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[usabaker]

Shoulder setback from primer force in rimless bottleneck cases is well-documented,

I only see reference to cases having a bullet in the mix. I do not see any reference on that forum that shows evidence that a primer in a case with no barrel restrictions can cause shoulder setback. OR I'm just missing it. Can you point me to the data to support primer force set-back with no restriction in the case mouth or chamber?

[Parashooter]

I only see reference to cases having a bullet in the mix. I do not see any reference on that forum that shows evidence that a primer in a case with no barrel restrictions can cause shoulder setback. OR I'm just missing it. Can you point me to the data to support primer force set-back with no restriction in the case mouth or chamber?

Author of post describes test with inert primers vs. live primers - no mention of bullet or powder for the test (which certainly wouldn't add much with inert primers) -

There have been basically the two theories regarding the cause; the firing pin blow theory and the primer theory. I ran the same tests with a fire formed case and inert primers; headspace was not changed. I then used the same fire formed case with live primers. In as little as two firings there was a measurable decrease in headspace. After five live primers the fired primer was noticeably backed out after firing. NOTE: this increase in headspace was with case taking LR primers.

If you really doubt that the effect occurs, this kind of test is something you can easily reproduce at home (just the live primer part needed). Cost is only a little labor and a few primers. Results vary depending on case/chamber surface condition, presence of lubricant, brass hardness, etc.

[usabaker]

If you really doubt that the effect occurs, this kind of test is something you can easily reproduce at home (just the live primer part needed). Cost is only a little labor and a few primers. Results vary depending on case/chamber surface condition, presence of lubricant, brass hardness, etc.

I didn't say I doubt it, I was just looking for the data, actual controlled engineering tests. I found the text in the forum you sighted. He does not describe the tests or test result data such as cartridge type, primer type, caliber, bore size, etc... in paragraph two. In fact, the author writes that the test were squib loads; which is supported by his statements, in paragraph three and four where he writes about squibs and the bullets he uses. The problem with what Mr. Ginson wrote is the lack of test cases and results that describe his test in detail or as primer only test, which leaves what he wrote up to the interpretation of the reader and not set facts.

On another note, if we -assume- tests were primers only with no cartridge obstruction or barrel obstruction and we take what Mr. Ginson wrote, "In as little as two firings there was a measurable decrease in headspace " This would mean that discharging "popping" primers once would not cause any problems since it took at least two firings to get measurable change in headspace, for which he offers no data. What is measurable? .010? .001? .0001? saying just 'measurable' is subjective.

Just do not see any data that supports shoulder setback on a primed cartridge with no cartridge obstruction or barrel obstruction when fired. But I do find this fascinating and will more than likely run test to see what results I get. I spent 23 years of my life and a test engineer so this will be interesting.