Case Separation, need input


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amlevin
July 7, 2011, 12:14 PM
The other day I was at the range shooting some new loads for my AR-15. The rifle is a custom build with a Bushmaster Dissipator 16" barrel.

The range requires single round loading so I was able to inspect each case as it was ejected. One case showed a perfect ring around it, about 5/8" above the base (about the length of a 9mm case from the head). The "ring" was like a scribed line. Closer inspection showed this "line" to be a hairline crack forming a perfect circle around the case.

When I returned home I took a pair of pliers and squeezed the case above the line. As expected the case fractured and revealed a "groove" that extended all the way around the case. It surprised me as it looked just like the groove that appears near the base after a case has been reloaded several times.

The load was a 55 gr Hornady V-Max bullet over 25.3 gr of H4895. The primer showed absolutely no pressure sign and even had a substantial radius on its edge remaining. No cratering of the firing pin or extractor mark. The round fired and sounded just like all the others. The Case was LC 01 marked and this was the first reload. It had the original crimped primer when I processed it. None of the remaining loads showed any similar signs.

Any ideas or suggestions? This seemed to be a strange occurrence to me as most separations have always started just above the web the few times I've had one. Why this far up the case with the same type internal "groove"?

A curious mind wants to know.

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brickeyee
July 7, 2011, 12:19 PM
Why this far up the case with the same type internal "groove"?

It is the spot in the chamber that the brass is thick enough it does not expand completely.

It would appear you are full length sizing each time, and while this allows the ammunition to fit any gun, it also results in stretching of the brass with every firing.

amlevin
July 7, 2011, 12:47 PM
It would appear you are full length sizing each time,

Since this was the FIRST reload ever for this case would you recommend any other method? I know it was the first reload as the case still had the military crimp on the primer when I received it. I had to swage the primer pocket before reloading.

Also, did you note where I said the cracking was located? Definitely not the thickest part of the case by any stretch of the imagination. About another 1/16" farther up or so and it would be in the mid-point of the case wall between head and shoulder.

DILLONHELP
July 7, 2011, 01:29 PM
Do you have a headspace case gage? If not, I urge you to obtain one. This shows you the headspace of the fired brass, as well as how your size die is adjusted. High pressures are not typically the cause of case separations. It could be the brass was fired in an M-249, which has a generous chamber, or your size die is down too far, or the rifle in question has a generous chamber; or any combination thereof. A case gage will help you to narrow it down.

MtnCreek
July 7, 2011, 01:51 PM
As said, I would set the sizer w/ a case gauge. I would also check fired cases with a case guage to see how far out of wack they after firing.

It's probably just a bad case.

Edit: I just realized The post above mine is from Dillon's CS. Do What He Said!!!

rg1
July 7, 2011, 02:02 PM
I've bought a few 1000 round batches of once fired military surplus brass. I always use the bent sharpened wire "feeler" method to feel for stretch marks or cracks in these once fired cases. I've usually found one to three questionable cases per 1000 rd. lot. I have found a few cases with definite stretch marks and these were once fired. I suspect you just ran into a case that was stretched and sizing and firing the second time caused a failure. I do recommend a case headspace gauge to set up your sizing dies to only push the shoulder back a measured amount. I have both Hornady's Headspace set and an RCBS Precision Mic.

Walkalong
July 7, 2011, 02:33 PM
It isn't a pressure thing. Case head separation happens when the shoulder is pushed back too far. Each time the case is fired the FP pushes the case forward until the shoulder stops it. Then the pressure expands the front half of the case gripping the chamber walls sealing the chamber. Then the rest of the case stretches until the case head stops at the breechface. The stretching occurs right where you describe. Excessive headspace can cause it in one firing, but usually it takes numerous firings in a normal chamber with proper sizing. How many depends on how much pressure, and how much headspace there is. (Extra room for the case to stretch) While technically headspace is machined into the chamber and is a set number, we can induce artificial headspace if we push the shoulder back too far. There are numerous ways to check to see where our shoulder is on fired brass and where we push it to sizing.

http://www.thehighroad.org/showpost.php?p=6270171&postcount=1


After each firing one should check inside the case for signs that case head separation is close to happening. You can use something as simple as a paper clip.

http://www.thehighroad.org/attachment.php?attachmentid=103714&d=1250726722

If you can feel the rut with a tool, you need to scrap the case.

243winxb
July 7, 2011, 04:15 PM
None of the remaining loads showed any similar signs.
I know it was the first reload as the case still had the military crimp on the primer when I received it. The brass can be damaged on the very first firing with factory ammo if the rife has excessive headspace or the loaded rounds shoulder was pushed back to far on manufacture. The case can separate any where in the body. Buy an RCBS Precision Mic, much better than a case gage.

amlevin
July 7, 2011, 06:06 PM
Thanks "243winxb". This is along the lines of what I expected.

I was surprised as this is the first case failure I've had with any 5.56mm cases in over 10,000 pieces of once fired I've loaded.

For other responders,

Yes, I have a case gauge (wilson) and use it to set the sizer die. No, my chamber is not "generous" as I have measured it.

I also have a Hornady headspace gauge and have compared cases fired in this rifle with those from the sizer. Usually not more than .002" or so difference between fired and freshly sized.

Thanks for all the responses, based on the fact that this is not a regular occurance I will write it off to a "bad case".

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