Head spacing. Case mouth vs extractor?

Discussion in 'Gunsmithing' started by kyhunter, Dec 6, 2016.

  1. kyhunter

    kyhunter Member

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    Just a question most likely addressed already but here we go. Since the case in a pistol round is held tightly against the striker "bolt" face with the extractor doesn't it really head space on the extractor?
     
  2. Oldhand

    Oldhand AKA Rawhidekid! Lifetime Supporter

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    No, a properly sized case will be pressing against the shoulders at the end of the chamber.
     

  3. SavageGuy

    SavageGuy Active Member

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    ^Yeah, that's your answer. A good question though
     
  4. phideaux

    phideaux Well-Known Member Lifetime Supporter

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    Here ya go,

    383574570head.gif

    Jim
     
    Last edited: Dec 7, 2016
  5. spikedriver

    spikedriver Active Member

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    Ok...I never thought about headspace except for necked cartridges. So for a straight walled case, if it has a rim like the .357 mag it headspaces on the rim. But a rimless cartridge, like a 9mm or .45 ACP, headspaces on the mouth. Is that correct? I've never handliaded a rimmed cartridge so I never even considered it...
     
  6. phideaux

    phideaux Well-Known Member Lifetime Supporter

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    headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge, to the face of the bolt.

    Here's a typical 45acp.
    [​IMG]
    A rimless straight walled case lacks a shoulder or tapered neck/case walls. This type is particularly popular for pistol ammunition (9×19mm Parabellum, .40 S&W, .45 ACP, etc.), although there are also a number of straight-walled rimless rifle cartridges (.50 Beowulf, .450 Bushmaster). Lacking a rim or a case neck to headspace off of, the straight walled rimless cartridge instead headspaces off of the case mouth, the forward rim of the case, where the thickness of the case wall fits around the bullet shank. This provides a slight ledge, similar to a belted case, only at the forward extremity of the case rather than at the rear. The chamber of the gun is bored out to the same diameter of the case, with a slight step down in size where the case ends. When chambered, the cartridge fits into the chamber until the case mouth is pressed against this step, preventing the cartridge from being inserted further. This ammunition type must not be roll crimped as the rounded edge of the case mouth will not engage safely with the stop ledge, this is why a light taper crimp is often used if considered necessary. Crimping around the case wall can also effect the overall length of the round, meaning that the headspacing will no longer be accurate; overall length must be kept within tight tolerances in this type of cartridge.



    Jim
     
    Last edited: Dec 7, 2016
  7. spikedriver

    spikedriver Active Member

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    Rog. Understood, thanks Jim.
     
  8. MagBlackhawk

    MagBlackhawk Patriot

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    ^ ^ True, (Straight walled rimless cartridges).

    And yet I have dies for rimless (auto-loader) cartridges that came with a roll crimp / bullet seat die.
    The last dies I bought were for 9mm. I got the four die set that comes with a taper crimp die.
    Guess what, the bullet seat die has a roll crimp built in.... It's no wonder there is confusion about this.
     
  9. kyhunter

    kyhunter Member

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    What I have found in measuring 45 acp empties is the case length is typically shorter than the drawings and the chamber per se. What I have observed in taking apart the slide is when the case is under the extractor it it tightly held to the bolt face ( sorry not sure what it is called in a striker or 1911 pistol) by the extractor and doesn't seem like the mouth of the case is against the undo of the chamber at all. Just what it seems. Ky
     
  10. phideaux

    phideaux Well-Known Member Lifetime Supporter

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    By empty cases , are you referring to once fire cases or NEW cases ?




    Jim
     
  11. kyhunter

    kyhunter Member

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    I am referencing fired cases.
    Read a post recently where a pistol would only fire consistently and not have light primer strikes when using 45 acp cases that were within .010 of the length on the std drawing for that case. I frequently measure fired cases well below this amount up to .015 or so below the drawing.
    So seems the case length is where the head space is measured from but also seems the extractor is what holds the loaded case against the bolt. KY