How exactly does a rimfire cartridge function?

Discussion in '.22-Rimfire Forum' started by hkruss, Dec 11, 2011.

  1. hkruss

    hkruss Active Member

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    I've never really understood how they function.

    On a centerfire rifle round, you look at the base and you can obviously see the bottom of the primer right there. Firing pin strikes it and it goes boom.
    Rimfire has no obvious primer, and the firing pin strikes the edge to make it fire.
    So I guess my dumb question is, is the entire base of a rimfire, a primer of sorts? I know that something has to initiate the explosion of powder in the case. I would like to know what that something is.


    So when the pin hits the edge of the round on a .22, what exactly causes it to fire?



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  2. flintlock

    flintlock Well-Known Member

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    The priming compund is in the rim of the case and is ignited by the strike of the firing pin.
  3. Alpo

    Alpo Well-Known Member

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    Maybe this'll help.

    Here's a picture of a modern centerfire cartridge case (the one on the right), sectioned. You can see the solid rim and the pocket where the primer sits, and the flash hole where the flame from the exploding primer gets in to set off the powder.

    [​IMG]

    Compare that to these sectioned rimfire cases. Notice that the rim is not solid, nor is it a separate portion of the case. The case is just folded to make the rim - the rim is hollow. A liquid priming mixture is dropped into the case, and the case is spun, allowing centrifugal force to shove the mixture into the hollow rim. Once it dries, the powder and then the bullet are added.

    [​IMG]

    When fired, the rim is crushed between the firing pin and the edge of the chamber, igniting the primer which set off the powder.

    Some of the older rimfire guns had a firing pin with two strikers. It hit both sides of the case at the same time, helping to ensure reliable ignition. Like this Joslyn rifle case. Others had a huge firing pin, that crushed a larger area of the rim, such as this Spencer case. Same reason - better chance of making it fire.

    Attached Files:

  4. hkruss

    hkruss Active Member

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    Thanks Flintlock.
    I was about to ask you some more specifics about your answer, then I saw Alpo's response which headed off most of my subsequent questions.

    Alpo, thanks for taking the time to post the detailed answer. I appreciate it!

    A couple of follow up questions.
    Any idea what the make-up is of the liquid priming mixture?
    Second, does the priming mixture explode simply as a result of being crushed abruptly? Kind of like, if you dropped a bottle of nitroglycerin, it would explode even though there was no spark to set it off.
    I guess I'm thinking in terms of there always having to be a spark to initiate an explosion, but I'm guessing in this case, it is simply a chemical reaction to a force being rapidly introduced (i.e., the firing pin crushing the rim).

    Am I far off in my guess?

    Thanks.


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  5. Alpo

    Alpo Well-Known Member

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    Yes, all it needs is HIT. Same as caps, for a cap pistol. The hammer of the pistol hits it, and it goes pow. Or you could put the whole roll down on the sidewalk and hit it with a hammer, and it goes POW.

    In a centerfire primer, the priming compound is in the primer cup, and there is an anvil pressed against it. The firing pin hits the cup, crushing the priming compound between the inside of the cup and the anvil, and it explodes.

    It's the same system as a percussion cap, for a muzzle loader. The cap is held on the gun's nipple. The hammer hits it, crushes it between the hammer and nipple, the mixture explodes from the percussion of being struck, and flame goes down the hollow nipple to ignite the powder.

    I'm sure you've seen these things. Work the same way - percussion.
    [​IMG]

    While trying to find pictures, I found this website. Should explain it all.

    http://www.reviewsofthings.com/guns/how_does_a_primer_work.html
  6. todd51

    todd51 Well-Known Member

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    Excellent Alpo.
  7. hkruss

    hkruss Active Member

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    Thank you Sir!


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  8. JUNKKING

    JUNKKING Active Member

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    I had a friend ask me this just a few days ago. I explained it to him as best as I could but nothing like you just posted Alpo. I never knew the rim was hollow. Thanks for the post. I will send a link to my buddy so he can actually see how it works.
  9. da357mag

    da357mag New Member

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    Learn something new everyday! I didn't know what the primer was actually made of!:eek: interesting!:D Doug
  10. Albtraum

    Albtraum Active Member

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    I was never sure about this until now, especially the liquid primer part (liquid primer... sounds like paint :rolleyes:) Thanks!

    Do those cross sections have the accurate amount of powder they would have? (or half?)
  11. carver

    carver Moderator

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    Once the primer material is in the bottom of the case (rim fire), the case is spun to distribute the material to all spaces in the rim. Sometimes there are gaps.
  12. Alpo

    Alpo Well-Known Member

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    Which was the reason for the double firing pin, and the extra-large firing pin.
  13. carver

    carver Moderator

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    And the reason why some .22 rounds don't fire the first tim they are struck by the firing pin!
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