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Case stretching

This is a discussion on Case stretching within the Ammunition forums, part of the M14 M1A Forum category; Generic question... Is case stretch as much of an issue in the AR-10 platform as much as it is in the M14 platform? I understand ...


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Old March 16th, 2017, 09:06 AM   #1
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Talking Case stretching

Generic question...

Is case stretch as much of an issue in the AR-10 platform as much as it is in the M14 platform? I understand the differences in the operating system, but I've always wondered about it, and this is for the 7.62mm AR, not 5.56mm.


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Old March 16th, 2017, 09:14 AM   #2
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Quote:
Originally Posted by charlie98 View Post
Generic question...

Is case stretch as much of an issue in the AR-10 platform as much as it is in the M14 platform? I understand the differences in the operating system, but I've always wondered about it, and this is for the 7.62mm AR, not 5.56mm.
Good question, I've had the same quandary myself?

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Old March 16th, 2017, 11:13 AM   #3
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Following. I admittedly didn't realize there was a difference...assuming M14 brass stretches more?

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Old March 16th, 2017, 11:17 AM   #4
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I would think it would be determined buy the chamber of each individual barrel. Some are tight some not so much. I don't think you could say 100% one platform stretches them worse than the other.

My 2 cents

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Old March 16th, 2017, 01:50 PM   #5
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I've not noticed much of a difference, but I'm not too anal about segregating brass.

M1 Carbine wins the case-stretching contest in my little herd . . .

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Old March 16th, 2017, 02:21 PM   #6
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Tell me I am wrong, but the M14 has 2 locking lugs (yes, large ones) but the AR family has 6-8 lugs to contain the pressure?

I would expect less case growth on a AR.

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Old March 16th, 2017, 02:30 PM   #7
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I was told that an M14 unlocks faster than an M16, so I would assume it is more susceptible to case stretching. I have absolutely no empirical evidence to back this up, other than my fired cases.

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Old March 16th, 2017, 03:02 PM   #8
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charlie98 asked:
Quote:
Is case stretch as much of an issue . . ?
The case stretches to fill the chamber. If the chamber is long and/or the cases are sized too much, then the fired cases begin to shows signs of incipient separation. I had a small batch of brass that I fired a dozen times: necks become brittle and crack, and primer pocket depth begins to decrease.

My M1A beats up on the base of the cases, but I don't get stretching (the bright line around the case) with minimal sizing.

Go to this post for a pic of incipient separation: (#8 by Girth)

What does this look like? Case separation?

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Old March 16th, 2017, 03:28 PM   #9
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No problems with excessive case growth on my ar10s but they are not entry models.

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Old March 16th, 2017, 05:30 PM   #10
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1) Case stretch is caused by the difference between the cartridge case headspace (unfired) and the chamber headspace. Any two rifles with similar difference between these two measurements will see similar case stretch.

2) The M1/M14/M1A has slow initial extraction, if you look closely while turning the bolt on your M1A you will see the the bolt is cammed backwards about 0.075 inch as it rotates. This allows the extractor to slowly break the case free from the chamber during the high pressure portion of the extraction cycle. The AR 15, AR10 and Johnson multi-lug bolt has no slow initial extraction. The bolt rotates 22-1/2 degrees then starts to move backwards. The extractor must snatch the case free from a dead standstill. This is much rougher on the case rim.

3) Unlocking time for the M14 is roughly the same as the AR-15, and the residual chamber pressure at the start of extraction is roughly the same.

4) The number of lugs has nothing to due with "how well" it contains the pressure. You either have enough shear area to resist the chamber pressure or you do not. The FAL has one locking lug, the M1, M60, and M14 two, the AR has seven (except Armalite made ones, which have 6). If you look, you will see that as the number goes up the size goes down.

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Old March 16th, 2017, 06:01 PM   #11
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Just my 2 cents, my .243 ,as we all know is a necked down .308, and I have found out thru the years that my .243 cases stretches much more than my .308 or even my 30-06. I think case stretching may be more related to how much of an angle the neck is, with all thing being the same; but like wasted ammo said, it could be just how the barrel is chambered. Like I said just mine 2 cents.

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Old March 16th, 2017, 06:49 PM   #12
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Quote:
Originally Posted by lysander View Post
1) Case stretch is caused by the difference between the cartridge case headspace (unfired) and the chamber headspace. Any two rifles with similar difference between these two measurements will see similar case stretch.

2) The M1/M14/M1A has slow initial extraction, if you look closely while turning the bolt on your M1A you will see the the bolt is cammed backwards about 0.075 inch as it rotates. This allows the extractor to slowly break the case free from the chamber during the high pressure portion of the extraction cycle. The AR 15, AR10 and Johnson multi-lug bolt has no slow initial extraction. The bolt rotates 22-1/2 degrees then starts to move backwards. The extractor must snatch the case free from a dead standstill. This is much rougher on the case rim.

3) Unlocking time for the M14 is roughly the same as the AR-15, and the residual chamber pressure at the start of extraction is roughly the same.

4) The number of lugs has nothing to due with "how well" it contains the pressure. You either have enough shear area to resist the chamber pressure or you do not. The FAL has one locking lug, the M1, M60, and M14 two, the AR has seven (except Armalite made ones, which have 6). If you look, you will see that as the number goes up the size goes down.
Reasons for my post are twofold...

I've had issues with case head separation using once-fired military brass in my Socom, in fact, I've had case head ruptures (traced back to a single headstamp.) I don't know how much I need to fret about head separation on my virgin brass; that being said, I've not loaded any of my original cases more than twice (3 firings.)

A forum friend of mine is getting an AR-10 (I don't know any particulars) and the question came up in my mind when I was giving him some hints on loading for the gassed .308.

Lysander's point #3 probably makes the most technical sense, even though we are talking AR-10, not AR-15... although I suspect it still is pretty close. His point #2 also seems relevant... I never thought much about the differences in the bolt camming out vs the carrier camming the bolt, but I can see now.

I guess I had it in my mind that the majority of traditional case stretch occurred at extraction, not at firing.

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Old March 17th, 2017, 03:13 AM   #13
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Quote:
Originally Posted by charlie98 View Post
Reasons for my post are twofold...

I've had issues with case head separation using once-fired military brass in my Socom, in fact, I've had case head ruptures (traced back to a single headstamp.) I don't know how much I need to fret about head separation on my virgin brass; that being said, I've not loaded any of my original cases more than twice (3 firings.)

A forum friend of mine is getting an AR-10 (I don't know any particulars) and the question came up in my mind when I was giving him some hints on loading for the gassed .308.

Lysander's point #3 probably makes the most technical sense, even though we are talking AR-10, not AR-15... although I suspect it still is pretty close. His point #2 also seems relevant... I never thought much about the differences in the bolt camming out vs the carrier camming the bolt, but I can see now.

I guess I had it in my mind that the majority of traditional case stretch occurred at extraction, not at firing.
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.


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Old March 17th, 2017, 05:49 AM   #14
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Great question charlie98 thanks from all us new guys ,and wow great responses I am always impressed at the knowledge base here but lysander you have really taken us to school kudos on your response with an honerable mention to shooter86314.

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Old March 17th, 2017, 06:29 AM   #15
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More than likely, since it was one particular headstamp, that the brass was on the hard side.
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.

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