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cases streatching - headspace?

 
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  #15  
Old 08-19-2013, 10:02 PM
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Re: cases streatching - headspace?

My logic says that if the primer has enough force to move the case it will be a against the bolt face. Not forward. Nor can I imagine the firing pin having any forward influence while firing a live round. The explosion pressure will vastly exceed the amount of influence from the firing pin. But I'm not an engineer.
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  #16  
Old 08-20-2013, 07:18 AM
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Re: cases streatching - headspace?

Rusty, I used to think like you are regarding what happens to primers and cases when they're fired. Then someone suggested I make some primed cases, some with and some without bullets, but all without powder, use a case headspace gauge to make before and after head to shoulder measurements on those .308 Win. cases. I did that.

Then I loaded 10 rounds of .308 Win. ammo using new cases starting with 44 grains of IMR4064 and some 165-gr. bullets. Each subsequent load's charge weight was 1 grain less. Case headspace on those new cases ranged from 1.627" to 1.629" and each was marked with its dimension for comparison after firing. Chamber headspace in the test rifle was 1.631". Loads with 44 down through 41 grains all had primers flush with the case head. Case headspace had increased to 1.630 for those loads. The 40 grain load had its primer sticking out about .002" and its headspace was 1.629" Each successive lesser charge down to 38 grains ended up with the primer sticking out a bit more as well as case headspace getting shorter. I stopped shooting after that; it's primer stuck out about .007" and case headspace was 1.625"; back .003" from its original 1.628" measurement. I didn't think it was necessary to shoot loads more reduced, but later shot the 37 grain load and it was the same as the 38 grain one.

My deductions from this test are as follows.

Firing pin drives case hard enough into the chamber shoulder to set the shoulder back a few thousandths. As the round fires, the front part of the case body holds the case against the chamber wall at that point as the back end of the case gets pushed back against the bolt face. Meanwhile, the primer gets pushed out of its pocket until pressure is high enough to have it pushed back in as the case head gets pushed back against the bolt face.

If there's not enough peak pressure in the case, the primer will be sticking out of the case head and it's headspace will be shorter than before it was fired.

Firing several primed .308 cases without powder or bullet had each one's headspace 2 to 3 thousandths shorter. All primers were still at their same depth in their pockets; about .003" below flush.

Fired 20 rounds of ammo without powder, but primed and with bullets seated. All had their shoulders set back a couple thousandths, primers and bullets still in place.

Readers can form their own reasons for the results of such stuff.
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  #17  
Old 08-20-2013, 06:12 PM
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Re: cases streatching - headspace?

Quote:
Originally Posted by Bart B View Post
Firing pin drives case hard enough into the chamber shoulder to set the shoulder back a few thousandths.
This is probably true with your the 20deg shoulders and high body taper of a 243/308, and possibly excess striking forces. It would be even more likely with a 270/30-06, both a solid standard in poor case design.
It doesn't happen with better designed cases with 30deg shoulders or more.
In fact, it doesn't even happen with smaller cases like a 223, even with a low shoulder angle of 23deg, because the shoulder is short.
Yes, anyone here can test this quick, and form their reasoning.

Quote:
Originally Posted by Bart B View Post
As the round fires, the front part of the case body holds the case against the chamber wall at that point as the back end of the case gets pushed back against the bolt face. Meanwhile, the primer gets pushed out of its pocket until pressure is high enough to have it pushed back in as the case head gets pushed back against the bolt face.
This is actually true, without qualification, and happens with all cases on firing.
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  #18  
Old 08-20-2013, 07:54 PM
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Re: cases streatching - headspace?

To be the devils advocate. The case walls blown against the chamber walls insure a lot of friction either forward or backward. (try pulling a fence post out of a hole :-) )

The amount of friction will depend on a lot of variables like how polished is the chamber wall, and /or how polished is the brass.

To say nothing of how much case lub is still on the case , point is, how totally free is the brass from any friction reducing substance e like the grease of your fingers etc.
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  #19  
Old 08-20-2013, 09:01 PM
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Re: cases streatching - headspace?

Rick, what you say is true.

All of which is why one should clean all the case lube off their cases after sizing them.

And don't polish cases to a mirror finish.

Have normal reamed chambers made; don't polish them bright and shiny.

The more thrust the case head has on the bolt face, the more force there is applied to the bolt's locking lugs.

There's an accuracy issue, too. If the bolt face ain't square with the chamber axis, previously fired cases from it will have their heads flattened to about the same angle. When both case and face high points align with each other when the case head slams against the bolt face, that off-axis force causes the barrel to whip in that direction. Shots don't go where they're aimed. Although this amounts to no more than 1/3 to 2/3 MOA error at most, that's enough to cause some folks to wonder why their loads don't shoot very accurate.
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  #20  
Old 08-21-2013, 08:56 AM
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Re: cases streatching - headspace?

So you're saying you don't polish the chamber, and don't tumble polish your brass to be bright and shiny?

Why?
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  #21  
Old 08-21-2013, 12:36 PM
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Re: cases streatching - headspace?

Quote:
Originally Posted by MudRunner2005 View Post
So you're saying you don't polish the chamber, and don't tumble polish your brass to be bright and shiny? Why?
Yes, that's what I'm saying.

Those smooth surfaces permit more case body sliding back wards pushing the case head harder against the bolt face. And the result may be more banana shaping of the case.

I don't need my chamber walls and cartridge cases to win beauty contests. Making 'em shiny and bright does nothing to the bullets' performance down range. Besides, nobody ever sees them when they're used to do their thing shooting a bullet out of the barrel.
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