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Thread: Flattened primers as a function of headspace

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    Moderator hunt_ak's Avatar
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    Default Flattened primers as a function of headspace

    After doing some reading on this, help me out and see if I understand this correctly.

    If the gap between the rear of the case and the front of the recoil surface is too great (round seated to far into chamber), then as the primer is ignited, due to its own pressure, it pops itself out of pocket. After the powder is ignited, the rear of the case (with the unseated already struck primer) smashes back into the recoil face of the slide, flattening the primer and possibly the case head as well.

    This happens because there is just too much room for the case to fly rearward willy-nilly without the aide of a recoil spring slowing it down. Is this correct?

    This is one of the reasons shooting .40 S&W outta a 10mm is a no-no also, right....just too much of a gap and the case now becomes a rearward projectile (though for a very short distance)...?

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    Member Vince's Avatar
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    This is one of the reasons shooting .40 S&W outta a 10mm is a no-no also, right....just too much of a gap and the case now becomes a rearward projectile (though for a very short distance)...?

    YOU found this out HOW?


    and i was hoping all the dumb stuff was done when i was your age...............(i still hope so)
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    Moderator hunt_ak's Avatar
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    Quote Originally Posted by Vince View Post
    YOU found this out HOW?
    No, I haven't ever done that, just trying to understand...

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    Excessive headspace allows movement of the case during firing. This can cause case stretching, case separation (ruptured case), and gas leakage. When the powder is ignited the base of the cartridge can move back while the sides of the case stick to the walls of the chamber. As a result the case can separate and rupture. If the bolt and receiver are not strong enough to contain and vent the blast you can at the very least damage the firearm or at worst you can injure or cause even greater harm to a bystander or yourself. Some mil-surp firearms are designed to handle a problem like case rupture. The ported holes on the side of Mauser bolts are an example of a design to vent off gases that may be inadvertently sent through the bolt to the rear of the firearm. If designs like this do not exist in the firearm you are shooting, then you could be in trouble.
    The wayward pressure and gas has got to go somewhere!

    Note that the above description is not my own, but from a very nice article posted on the surplus rifle web site link below….

    http://www.surplusrifle.com/shooting/headspace/index.asp
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    Quote Originally Posted by hunt_ak View Post
    After doing some reading on this, help me out and see if I understand this correctly.

    If the gap between the rear of the case and the front of the recoil surface is too great (round seated to far into chamber), then as the primer is ignited, due to its own pressure, it pops itself out of pocket. After the powder is ignited, the rear of the case (with the unseated already struck primer) smashes back into the recoil face of the slide, flattening the primer and possibly the case head as well.

    This happens because there is just too much room for the case to fly rearward willy-nilly without the aide of a recoil spring slowing it down. Is this correct?

    This is one of the reasons shooting .40 S&W outta a 10mm is a no-no also, right....just too much of a gap and the case now becomes a rearward projectile (though for a very short distance)...?
    I saw this earlier in the week but haven't had time to respond to it so here oes a short blurb.

    Your description in para one is accurate, lacking only technical terminology. There are many factors to effect this one of which requires the primer to actually unseat from the pocket and move back. Most factory loaded rounds won't do that, given primer sealant, etc. This may happen more with new brass handloads and then maybe only half will do it and leave much doubt with the shooter as he tries to analyze pressure signs. First in such a situation with a bottle neck rifle that operates at 56,000 psi we should be concerned, that pressure is above brass forming level and will reshape your brass for you. It could also lead to a case rupture which would be ungood. The case head would not be flatten from slamming back to the bolt face but would from rifle pressures.

    The scenario of 40 S&W vs 10mm isn't really valid, but we could see the same results as any excess headspace.

    I don't follow para #2. You're referring to semi-auto pistol, I guess. I think generally shooting 40 S&W in a 10mm chamber has to be considered taboo, but I've done it to see what happens. The chamber supports the 40 as well as it does the 10mm and it is the extractor of some guns that allows to work. Some types of guns wouldn't allow this to work.

    But in any case, We should expect the primer of the 40 to look different when fired in a 10mm chamber. I doubt that there is any greater risk from the behavior of the primer when doing this, but there is some risk. But there is some risk every time we shoot a gun with proper ammo in proper chambers.


    It has been written that the primer does not reliably tell pressure signs. That's true but it is also true that some handloaders can tell pressure form the primer, accurately. There is obviously more to it than just studying the primer. You must know how a normal primer looks when fired from that particular gun with certainty, then compare normal pressure primers with high pressure primers. I can also tell a cartridge case fired in a Glock, a Sigma, Kahr, 1911 or a SiG at a glance, I've studied a few thousand of them. It is an art not a science.
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    Quote Originally Posted by Murphy View Post
    I don't follow para #2.....
    Yeah, after I re-read it, neither did I...

    Went and got the G20SF and 1 ea. 10mm and 40 S&W. Took the barrel out and just stuck both cases in there and obviously saw that the 40 was too far down the throat.....and thats what I was referring to when I wrote that. I guess I just neglected to acknowledge the existense of my extractor , as thats whats going to be holding it against the recoil face (should be some more technical terminology for ya in here).

    Thanks for bearing with me, guys!!

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    Take a .40 and fire it and take pics so we can see what the differences look like.
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    I've seen a 9mm fired in a 10mm, that looked rather funny, but the 40 S&W looked like it had been fired out of any other 40 when it ejected from my 10mm

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