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Headspace Variance

 
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  #1  
Old 06-23-2010, 09:31 AM
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Headspace Variance

Hello All,

I have some fire formed brass that hasn't been put through a die yet and hasn't even had the primers removed.

I'm using a RCBS Precision Mic to measure the headspace and I'm getting a variance of 1.5-2 thousandths on the measurment between the brass.

How can that be? Shouldn't it be much tighter than that since it's fire forming to a chamber?

Tom
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  #2  
Old 06-23-2010, 10:17 AM
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Join Date: Jul 2004
Location: Texas
Posts: 5,496
Re: Headspace Variance

Quote:
Originally Posted by statjunk View Post
Hello All,

I have some fire formed brass that hasn't been put through a die yet and hasn't even had the primers removed.

I'm using a RCBS Precision Mic to measure the headspace and I'm getting a variance of 1.5-2 thousandths on the measurment between the brass.

How can that be? Shouldn't it be much tighter than that since it's fire forming to a chamber?

Tom
This condition is common for several reasons= One could be the measuring process it's self.

Measure the same piece of brass at least 4 or 5 times zeroing the instrument each time and wright down each reading, obviously if they are identical then this is not the problem.

Mose brass has slightly differences in hardness and thickness so the amount of spring back will vary plus the loads them selves are not exactly the same pressures and this can cause
the brass to measure differently.

Head space could cause this if it were poor allowing the brass to move differently under certain
conditions.

If the chamber is not clean and dry (No Oil) it can allow the round to move differently each time
it is fired.

I'm sure there are other reasons that this can happen and maybe the others can add to this.

J E CUSTOM
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  #3  
Old 06-23-2010, 11:14 AM
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Join Date: Jul 2009
Posts: 219
Re: Headspace Variance

I already checked the repeatability. That one was easy. It does repeat even with rotations of the brass. The brass generally ends up 1 thousands over Saami spec.

The chamber is clean as far as I can tell.

I weigh my charges to the 1/10 of a grain. So I don't think it's the powder.

Can neck tension cause the pressure changes that you're talking about?

BTW - All the brass I measured shot the same powder and the same bullet.

I'm shooting Nosler Custom Brass. Do you think that brass issues could really cause that much of a difference?

Should I sort my brass by the after fireforming headspace measure?

Thanks

Tom
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  #4  
Old 06-23-2010, 11:52 AM
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Join Date: Apr 2005
Location: Alaska
Posts: 3,439
Re: Headspace Variance

I wouldn't worry about it until you've fired the brass approx three times in your rifle. It can take several firings for the brass to more fully form to the chamber dimensions. Because as JE stated, brass is not that exactingly the same from case to case.

When I sent a resizing die in for customizing to my chamber, the machinist wanted my casings to be fired a minimum of three times in my rifle, in order to allow them to fully expand to best represent the dimensions of my rifle's chamber.

I don't know if you're full length resizing or not, but you should just resize the cases enough to barely bump the shoulder of the case back 0.001-0.002". If after firing your cases three or more times, you still have variable headspace measurements, only then would I begin to think about the magnitude of the differing measurements, and perhaps troubleshoot the source of the varying measurements.
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Old 06-23-2010, 12:50 PM
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Location: Mountians of SW NC, near Asheville
Posts: 1,596
Re: Headspace Variance

"How can that be? Shouldn't it be much tighter than that since it's fire forming to a chamber?"

Ideally, yeah. But we don't live in an ideal world and our cases are for sure not ideal. Actually, what you're getting is pretty consistant! All that tiny difference reflects is slight variations in the metal alloy. The molten metal mixture does vary somewhat and stray soft spots in the final product will stay expanded better than harder spots. And it will get slightly different as you take them thro a few cycles and it hardens even more.

There is no point in moving the shoulders back any further than they are now, they have already shrunk back a tad more than full chamber size. You WILL decrease case stretch if you leave them (neck sizing) or restore them (FL sizing) to where they are now.

Last edited by boomtube; 06-23-2010 at 05:02 PM.
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  #6  
Old 06-24-2010, 10:19 AM
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Join Date: Jul 2009
Posts: 219
Re: Headspace Variance

Thanks to all. I really appreciate the input.

I hate to say it but this issue is kind of bothering me.

Seems like the explosion of the powder should conform the brass to the chamber dimension each time the same way irregardless of the brass. Seems like one would easily overcome the other.

Thanks

Tom
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  #7  
Old 06-24-2010, 10:35 AM
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Join Date: Apr 2009
Posts: 183
Re: Headspace Variance

Have you tried depriming them and then measuring.
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