Incipient Case Head Separation

thump_rrr

thump_rrr

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Went to the range today to shoot some .22LR.
Since I wanted to anneal this batch of 100 .308 brass before reloading I thought I would shoot my last 11 rounds in my M305.
I loaded up the first 2 mags and shot them without issue from the prone position
The prone stations at our indoor range are elevated and made of plywood so you can hear the rounds hitting the plywood.
I loaded the last round into the mag and fired it and I heard that the case didn't sound the same as the others when it hit the wood.
This is what I found.

The other half was nowhere to be found. It is a Winchester case and the neck thickness is 0.013" and the body thickness is 0.028"
The case was only reloaded 2 times so it was on its 3rd firing.
I checked the other 99 cases with a paper clip and I didn't find any ring inside any of them.
Load data was 41.7gr of IMR4064 over a 168gr SMK with a FGMM 210M primer loaded to 2.800" OAL.
The case was sized for the headspace of my M1A National Match which is 1.631" but was fired in my M305 who's headspace is. 1.638".

I think this one is on me.
 
M305 have really larger chambers. Mine was .017 over go .308 go gauge. Brass fired in these oversized chambers stretches the brass A LOT.

Usually you only get a couple of reloads out of it.
 
Happened to me several times. Even with Lapua brass, I get max 6-7 loads, then signs of head separation appear.
I load my brass 4 times, then throw.
Nature of the beast with a fat chambered war pig.
 
JR86 said:
have you ever annealed that brass before? are you saying you think the headspacing was the problem here?
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I believe the difference between the headspace of the National Match and the M305 is where I think the problem is.
If I only size down the brass fired out of the M305 enough to properly chamber it should increase its lifespan.
I will from now on only use military brass in the M305 which should also help.

BeerBaron said:
The second case in your picture is showing signs of case head separation as well. Note the light ring in the same location as the first piece split.
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I see the light ring but it passes the dental pick test.
I would have figured that it would show some internal sign if it were about to separate.
 
This is an interesting discussion given the cost and scarcity of 308 brass.

I have had case head separation on the second firing of FC brass and almost retired the entire batch however I thought I'd see if they would hold up to one more firing (after checking them all for stretch marks and cracks - internal and external).

I've been doing this now for 6 more firings in my M305 and still don't see any signs of imminent failure.

I full size but have reduced shoulder bump to the maximum SAAMI spec. Would I be dumb to try neck sizing with a few thousandths of shoulder bump?
 
thump_rrr said:
I believe the difference between the headspace of the National Match and the M305 is where I think the problem is.
If I only size down the brass fired out of the M305 enough to properly chamber it should increase its lifespan.
I will from now on only use military brass in the M305 which should also help.


I see the light ring but it passes the dental pick test.
I would have figured that it would show some internal sign if it were about to separate.
Click to expand...

Thump_rrr........You cant always feel it with a pick or paper clip. Depending on the make of brass, some of it stretches over a larger area and is virtually un-feelable. May I suggest you section a couple cases to see exactly what is going on.
Yep military chambers and reloading don't always compliment each other. The military criteria is 100% reliability the first firing, after that they don't care, hence the oversize chambers..........full autos are even worse. I've seen first fired 308 brass from a full auto (don't know what rifle it was, sorry) that was showing incipient separation on more than 1/2 the brass after the first firing. Thought I'd struck a goldmine until a buddy told me they were from a full auto and to take a real good close look at them...........Damn......no gold mine there, just a brass slag pile I'm afraid.
 
American commercial SAAMI cartridge cases are not made to the same specifications as military "milspec" cartridge cases. Military 7.62 cases are thicker and made of harder brass than commercial cases. This is because these military cases are designed for larger diameter and longer headspaced chambers.

For what its worth a full length resized .303 British Winchester cartridge case lasted three firings in a Enfield rifle with the headspace set at .067. If you want your cases to last longer I would find some Canadian military 7.62 cases and forget firing Winchester cases that have some of the thinnest case walls in the industry.

Also these type rifles are "over gassed" and the bolt can start moving to the rear while pressure is still in the barrel. This causes the brass to stretch even more because the cartridge case is still gripping the chamber walls as the bolt starts moving to the rear. You could try a faster burning powder or reduce your loads to lower gas port pressure to help prevent this.

.308 Win Cartridge Guide
http://www.accurateshooter.com/cartridge-guides/308win/

RELOADING FOR THE MATCH M14 (read this carefully about case construction)
http://www.zediker.com/downloads/14_loading.pdf


m14chamber_zps93174da1.jpg
 
If you want to use something more accurate than a bent paperclip checking for stretching and thinning in the web area then get a RCBS case mastering gauge. This gauge can measure thinning in a thousandths of an inch and find stretched cases far sooner.

Below the case isn't levitating on my finger, the case is resting between a pointed bent rod and the gauge and reading case wall thickness.

RCBSCMG_zpsb95d3710.jpg


rcbsgauge_zps9306c010.jpg


Below in these two photos shows a factory loaded once fired Winchester .303 case, the case stretched .009 on its first firing in a Enfield rifle with the headspace set at just below .067.

IMGP5204-1_zps5590eee6.jpg


IMGP4521-1_zpsa603b8a2.jpg


Head clearance or the "air space" between the rear of the case and the bolt face is the distance the case can stretch. If you keep your shoulder bump to a minimum when full length resizing your head clearance will be approximately .003 and within the elastic limits of the brass. This means the brass will stretch under pressure BUT it will "spring back" to its original size and not stretch and thin.

HEADCLEARANCE-a_zps1a9a1011.jpg


Below an animated image of the cartridge firing and stretching to meet the bolt face. As long as you do "NOT" over resize the brass it can not stretch and thin when reloaded.

HeadClearance_zpsf30a3af1.gif


Below, examples of .308 brass fired in a "normal" SAAMI chamber fired in a Savage bolt action rifle and showing case life.

308fail-1_zps30d387ab.jpg


308fail2-1_zps3ca31f6b.jpg


NOTE: The second case above labeled Federal military gives a FALSE impression of our military 7.62 cartridge cases. Federal sell ammunition with 7.62 Lake City military cases that FAILED military requirments, case wall thickness, hardness, etc. These rejected military cases are not normal and failed sooner than cases that met 100% of milspec standards.
 
If you want to use something more accurate than a bent paperclip checking for stretching and thinning in the web area then get a RCBS case mastering gauge. This gauge can measure thinning in a thousandths of an inch and find stretched cases far sooner.
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Awesome post. Stuff like this is why I'm practically glued to this forum.
 
HeavyTread said:
Awesome post. Stuff like this is why I'm practically glued to this forum.
Click to expand...

You need to learn how to multitask, I watched Alex Bilodeau win the Gold for Canada last night on TV "AND" had my laptop open getting firearms information. But then again Canada has twice as many Olympic medals as we Americans have so I hope all your guns rust and a Polar bear chews your legs off.

Laugh2
 
Just a little research I did on my own

Well sunray...it all depends if the a chamber is as sloppy as your usual intellect.

there is a reason why there are two different GO/NO GO gauges for the cartridges.

my own little reaserch

Alright, measured some ammo I have kicking around.

.308 Go Gauge is 1.630

S&B 308 = 1.623
Fed 308 = 1.626

7.62x51 Go Gauge 1.635

South African 7.62 = 1.630
Portuguese 7.62 = 1.629
PPU 7.62 = 1.630
Click to expand...
 
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