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Case stretching

This is a discussion on Case stretching within the Ammunition forums, part of the M14 M1A Forum category; Originally Posted by charlie98 I documented it HERE. .. including my cutaways. Reading back through that thread, I realize I still hadn't figured it to ...


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Old March 17th, 2017, 10:27 AM   #16
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Quote:
Originally Posted by charlie98 View Post
I documented it HERE... including my cutaways.

Reading back through that thread, I realize I still hadn't figured it to be a bad lot of brass... I though it was something I had done or the rifle's fault until I did that cutaway.
The case separation in question:


Here is a stress model of the head of a cartridge case, it is not a 7.62mm, so the values are not going to be correct, the areas of high stress in a .243 Winchester case are going to be the same areas of high stress in all rimless cartridge cases, so we can tell a few things.




Compare the stress colors to the crack.

1) Your cases failed in the second lowest stress region of the head.

2) Your case failed at a sharp discontinuity created by a machining operation.

Without doing hardness tests on the head, measuring the exact pressure your load was working at, and other such things, we can still say that those case failures were the result of some manufacturing defect.

Unfortunately, for you, since these were resized, you have now become a part of the manufacturing process, as well as Lake City. Therefore, your set-up is also suspect.

If your neck expander /de-priming pin is set to low and bottoms out in the case, it is possible to stress harden the case head in this area. This would leave the case wall pliable, and resistant to the high stresses they are subjected to, but make the case head brittle and easily cracked when asked to expand. Especially, if they started life on the high side of the tolerance band.

I would check the reloading die set-up.

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Old March 17th, 2017, 06:54 PM   #17
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Quote:
Originally Posted by lysander View Post
1) Your cases failed in the second lowest stress region of the head.

2) Your case failed at a sharp discontinuity created by a machining operation.

Without doing hardness tests on the head, measuring the exact pressure your load was working at, and other such things, we can still say that those case failures were the result of some manufacturing defect.

Unfortunately, for you, since these were resized, you have now become a part of the manufacturing process, as well as Lake City. Therefore, your set-up is also suspect.

If your neck expander /de-priming pin is set to low and bottoms out in the case, it is possible to stress harden the case head in this area. This would leave the case wall pliable, and resistant to the high stresses they are subjected to, but make the case head brittle and easily cracked when asked to expand. Especially, if they started life on the high side of the tolerance band.

I would check the reloading die set-up.
Actually, no, those cases were all once-fired, processed by Scharch (Top Brass) and is the reason why I've sworn off brass processed by anyone but me. I bought a 1K case lot, the LC04 were the only ones that failed in that manner... none of the other LC, WCC, PMJ, etc, failed in that specific manner, and I chalked it up to a bad lot of brass (LC04, not the entire 1K lot of brass.)

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Old March 17th, 2017, 07:03 PM   #18
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Quote:
Originally Posted by charlie98 View Post
Actually, no, those cases were all once-fired, processed by Scharch (Top Brass) and is the reason why I've sworn off brass processed by anyone but me. I bought a 1K case lot, the LC04 were the only ones that failed in that manner... none of the other LC, WCC, PMJ, etc, failed in that specific manner, and I chalked it up to a bad lot of brass (LC04, not the entire 1K lot of brass.)
I, too, don't buy processed brass for that reason.

Web thickness varies from manufacturer to manufacturer and from lot to lot. When resizing, you should always check to ensure the expander/decapping head does not bottom out. Possibly, the people doing the resizing did not check in this case.


Last edited by lysander; March 17th, 2017 at 07:14 PM.
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Old March 18th, 2017, 08:51 AM   #19
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IMHO... with an AR10, you can easily make the bolt unlocking slightly retarded... simply by adding a heavier buffer or a Tubbs CWS.

You will only gain milliseconds... but that will be all you need.

This link helps explain the gains from the Tubbs CWS ( added weight to the BCG to retard unlocking. )

http://www.davidtubb.com/catalog/vie...on/cwsbroc.pdf

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Old March 18th, 2017, 12:39 PM   #20
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If you go from a 3-3/4 oz standard DPMS pattern carbine buffer to a 5-1/2 oz stainless steel buffer, you gain a lot . . . .

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Old March 18th, 2017, 12:47 PM   #21
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More than likely, since it was one particular headstamp, that the brass was on the hard side.

Case stretch is caused by by the case shoulder being pinned against the chamber shoulder by the chamber pressure and the case head and bolt moving backwards to the maximum chamber headspace distance. You can increase the case life by resizing the case so the cartridge headspace is near the chamber headspace. That's why they sell cartridge headspace guages that allow you to actually measure the distance from base to the datum diameter on the case.

After the third firing of your brass, pick twenty cases at random, and check for excessive stretch visually and with a bent paper clip.
Thanks for that cross cut casing image. That was an eyeopener. I get about 4-5 firing from casings in my M1A before I see the ring of doom. Interesting to see what's really going on on the inside.

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Old March 18th, 2017, 02:14 PM   #22
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Quote:
Originally Posted by lysander View Post
If you go from a 3-3/4 oz standard DPMS pattern carbine buffer to a 5-1/2 oz stainless steel buffer, you gain a lot . . . .
I thought the tungsten was the heaviest?

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Old March 18th, 2017, 02:30 PM   #23
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Originally Posted by Phil McGrath View Post
I thought the tungsten was the heaviest?
The standard DPMS .308 carbine buffer (2-1/2 inch long) has an aluminum body and two tungsten weights and weighs about 3-3/4 oz. The Armalite pattern .308 carbine uses a standard H3 buffer weighing 5-1/4 oz.

Generally speaking, the big ARs benefit from the heavier buffers.

In order to get the heavy weigh in the 2-1/2 inch long buffer, you have to use something denser than aluminum, since you're maxed out with the tungsten weights. They are steel with a pair of tungsten weights. Any steel would do, but stainless is, well, stainless.....

A number of places sell them at a number of prices, ranging from $50 to $150


Last edited by lysander; March 18th, 2017 at 02:43 PM.
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