Explaining Headspace

Discussion in 'The Ammo & Reloading Forum' started by Insulation Tim, Jun 6, 2010.

  1. Insulation Tim

    Insulation Tim Well-Known Member

    Oct 29, 2008
    I have seen this term in a number of manuals, Headspace of a .45 ACP cartridge, which headspaces off the case mouth but have never understood what it meant.

    Can someone explain it to me, in understandable terms, what it means and specifically its relevance to reloading .45ACP.

    Thanks in advance.
  2. Alpo

    Alpo Well-Known Member

    Feb 3, 2007
    NW Florida
    Headspace is the distance from the head of the cartridge to the face of the bolt. If there is not enough room, the gun won't function. The bolt won't close, or (in the case of a revolver) the cylinder won't turn all the way. This is "too little headspace".

    When the cartridge is fired, before the bullet starts to move (remember the law of inertia - whatever is at rest tends to stay at rest, and the more mass it has, the harder it is to put into motion) the lighter brass case first expands out until it hits the chamber wall, and then it is slammed backwards until it hits the bolt (or, in the case of a revolver, the recoil shield). Once it has done that, the case can't move anymore, and the only thing that can move is the bullet, so it, finally, starts to move forward. If the distance between the head and the boltface (or recoil shield) is too great (this is referred to as excessive headspace) the case has farther to go before it stops, which allows it to build up more speed and more pressure before it hits, which can cause damage to the gun. Also, if the headspace is excessive, the firing pin may not be long enough to pop the primer, and you have misfires.

    Cartridges "headspace" in different areas, depending on the design of the cartridge.

    Rimmed cartridges headspace on the rim. That's why you can use both 38 and 357 in a 357 revolver. The 38's being shorter does not matter, since the rim is in the same spot on both rounds.

    Belted magnums headspace on the belt. They work the same as a rimmed round, in that if the case is a little too short, it doesn't matter, since the belt is always in the same place.

    Bottlenecked rimless rounds headspace on the shoulder. Like with the belted cases, it doesn't matter too much if the overall length is a little short, as long as the shoulder has not been blown forward or pushed back.

    The last group is the straightwalled rimless cartridges. They headspace on the mouth. 45 ACP, 9mm, 40 S&W, etc. If you look in the chamber of most modern pistol rounds, you will see a little "step" in there. This gives the case mouth somewhere to sit. Without it, since the case is rimless, the entire cartridge could fall through the chamber. Because they headspace on the mouth, overall length of the cartridge is critical. Too long and the bolt won't close (too little headspace) and too short and the cartridge goes too far forward into the chamber (excessive headspace).

    Some of the older revolvers don't have this step in them. The cylinder is bored straight through. There is no need for the step, since they headspace on the rim. Most modern revolvers have this step, though, but it is more of a safety issue than for headspace. The step will not allow you to chamber a 357 in a 38 special. Without the step, one might fit, with disastrous results.

  3. cycloneman

    cycloneman Well-Known Member

    Dec 16, 2008
    TRIGGERJA, CharlieHorse and greggs like this.
  4. Alpo

    Alpo Well-Known Member

    Feb 3, 2007
    NW Florida
    Just in case you didn't know, the head of the cartridge is the back end. The front, where the bullet is, is the mouth, while the back, where the markings are, is the head. That's why the markings are called "headstamps". :)

    Many people refer to bullets as "heads", or "bullet heads". They are wrong.
    oldfartrr likes this.
  5. muddober

    muddober Active Member

    Sep 19, 2008
    Carson City Nevada
    Alpo, I must admit you did a great job of explaining headspace but you must admit cycloneman also did a great job of simplifying it.

    TRIGGERJA likes this.
  6. Insulation Tim

    Insulation Tim Well-Known Member

    Oct 29, 2008
    Alpo...thank you very much. I also never knew that the cartridge end with the primer is the "head". Even at 63, I learn something new everyday....or at least try to.
  7. Terry_P

    Terry_P New Member

    Mar 23, 2008
    So it isn't where you put the toilet?


    Thanks as I wasn't real sure either but have a good understanding now.
  8. Alpo

    Alpo Well-Known Member

    Feb 3, 2007
    NW Florida
    Y'all are quite welcome. Glad I could help.
  9. JLA

    JLA Well-Known Member

    Feb 26, 2007
    Heart Of Texas
    alpo is a most useful asset to the forum;)
  10. JohnTheCalifornian

    JohnTheCalifornian Member

    Jun 12, 2009
    Should be thrown up top for a sticky. Good info for people to know.
    oldfartrr likes this.
  11. LDBennett

    LDBennett Well-Known Member

    Dec 20, 2003
    Hesperia, CA
    There are NO STUPID questions but..... this is the reloading forum and it is assumed that people who come here care about reloading. Such people should read their reloading manuals. All the above info is in the reloading manuals introductory chapters. I urge all to read their manuals, and re-read them until they thoroughly understand at least the terms if not the process and the "why's" of those processes. The stickies above also are a good introduction if you at least understand some of the terms.

    Those who come here to discuss commercial ammo (and that is just fine) might gain something by buying an reloading manual to get a better understanding of even commercially made ammo. I recommend the Hornady manual as it is very good at explaining the "how it works" of ammunition.

    An informed reloader/shooter is at the first step of safety for reloading and shooting.

    oldfartrr likes this.
  12. accident

    accident Well-Known Member Supporting Member

    Sep 11, 2009
    middle GA
    As always,good responses from Alpo and LD.Thanks guys for being such an asset to the forumn.Couldn't have done w/out ya'lls help.You have helped me,and i think i speak for others,many times.THANKS,Joe
  13. DGG!

    DGG! New Member

    Apr 6, 2010
    Don't you have to moon the .45 acp to get it to headspace in a revolver?
  14. Alpo

    Alpo Well-Known Member

    Feb 3, 2007
    NW Florida
    Negative. They will headspace on the little lip, just fine. Howsomever, without the moon clip you have to either punch 'em out from the front with a pencil, or pick 'em out from the back with your thumbnail or pocketknife. The moon clip's real purpose is to let the ejector star push 'em out.

    Very early Colt 1917s did not have the lip, and need the clips to work. Webleys don't have a lip, so if you have of them that has been shaved, it also requires clips. Smith, however, has had the lip from the beginning, and Colt went to it within about the first six months of production.
    Last edited: Jun 9, 2010
    oldfartrr likes this.
  15. JLA

    JLA Well-Known Member

    Feb 26, 2007
    Heart Of Texas
    or you could buy auto rim brass and have mini .45 colt cartridges that have a really thick rim (.090" compared to .060" of a normal revolver cartridge)
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