Headspace Variance

Discussion in 'Reloading' started by statjunk, Jun 23, 2010.

  1. statjunk

    statjunk Well-Known Member

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    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
     
  2. J E Custom

    J E Custom Well-Known Member

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    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
     

  3. statjunk

    statjunk Well-Known Member

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    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
     
  4. phorwath

    phorwath Well-Known Member

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    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.
     
  5. boomtube

    boomtube Well-Known Member

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    "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: Jun 23, 2010
  6. statjunk

    statjunk Well-Known Member

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    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
     
  7. learning

    learning Well-Known Member

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    Have you tried depriming them and then measuring.
     
  8. statjunk

    statjunk Well-Known Member

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    Wouldn't make a difference as the precision mic has a large hole at the bottom and it makes no contact with the primer.

    Tom
     
  9. boomtube

    boomtube Well-Known Member

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    "Seems like the explosion of the powder should conform the brass to the chamber dimension each time the same way irregardless of the brass."

    It does, fully. But, you're measuring fired cases that have expanded and shrunk back a tad and your gage is showing you that some tads are greater than others; it's a good tool. You will also find simular differences after you ever so carefully FL size your cases, the shoulders just won't be at the same place each time, especially if you get sloppy and inconsistant with the press lever. Dealing with those small variations is part of life for reloaders.

    Depriming before measuring will make no difference at all with that gage.
     
    Last edited: Jun 24, 2010
  10. statjunk

    statjunk Well-Known Member

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    Holy $hit. Never thought about the brass cooling in the chamber. Man this is why I love reloading. Always something to learn.

    Thanks for the lesson.

    Tom
     
  11. Mikecr

    Mikecr Well-Known Member

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    IT'S SPRINGBACK
    Seriously, you need to pick up a basic How-To-Reload book & read.
     
  12. coyotezapper

    coyotezapper Well-Known Member

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    +1 on brass variations and springback could be one or both. How are you loading your fireform loads?? I have fireformed a couple of Ackley cartridges and I use the heaviest bullet for caliber I can find and seat it on the lands and use the fastest powder that I can find data (parent cartridge) for, for that bullet. Faster powder and heavy bullets (more bearing surface) will create more pressure and give you better finished brass. I have had no problems in fireforming hundreds of cases using this method. And yes I have taken all critical measurements after fireforming and they have been almost perfect. Quality of brass does help. Steve Timm a former gunwriter for VHM helped me out on my first Ackley and I have been using this method ever since.
     
  13. statjunk

    statjunk Well-Known Member

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    Thanks Mike. Really appreciate your insight. I have read a reloading manual. To be specific Lyman 48. Just because you read a reloading manual doesn't mean you're going to remember everything you've read. Especially when you've read it a while ago. I could probably use a re-read but that it's going to be a while.

    Tom
     
    Last edited: Jun 27, 2010
  14. statjunk

    statjunk Well-Known Member

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    Wanted to provide some feedback on this issue. I was reloading for a friend when this issue came up. He has Federal brass from a 300WM. All his own fire formed cases with factory bought ammo.

    I've never seen this issue come up in my rifles. I only use Nosler and Winchester brass. Never more than .001 variance.

    I wonder if it's the powder or the brass causing this.

    Either way I'll stay away from federal.

    Tom