Large flash holes - dangerous?

Discussion in 'The Ammo & Reloading Forum' started by Alpo, Jun 11, 2009.

  1. Alpo

    Alpo Well-Known Member

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    When you fire a centerfire cartridge, several things happen, in order.

    The primer pops, shooting a jet of flame though the flash hole. The flame ignites the powder. The powder burns, producing high pressure gases. The gases are pushed by through the flash hole, their pressure being increased by the venturi effect of the small hole, and push the primer back. The case walls of the cartridge expand until they hit the chamber wall. The cartridge case is slammed back against the boltface/recoil shield, reseating the primer. Finally, when everything else that can possibly move has moved, the gas pressure overcomes the bullet's inertia, and starts it on its trip down the barrel.

    If you shoot a blank, the same thing happens, up to a point. The gases back out the primer, but without a bullet blocking the front, the case does not get slammed back, and the primer does not reseat.

    In a rifle or an automatic pistol, this is of no concern. With a revolver, though, the backed out primer can prevent the cylinder from moving, either to turn or to come out of the frame for reloading.

    The solution, I've always heard, is to drill out the flash hole. If you increase the flash hole size, until there is just enough bottom of the primer pocket left to hold the primer, that gets rid of the venturi effect, and the gas pressure is not high enough to back out the primer. Okay. That makes sense.

    Then I hear, "alway mark the case, like with a file-notch on the rim, so that you do not accidentally use these cases for live rounds. This is very dangerous". I always accepted that as true.

    A few years ago, Winchester came out with some new ammo, designed for indoor ranges. They called this stuff, "Non-Toxic", and it did not use lead in the primer. This new primer was, apparently, not as hot, because Winchester made the cases with a huge flash hole (45 ACP, standard Large Pistol primer pocket size). I ended up with a couple hundred of these, and wrote Winchester, asking if these were "one-shot disposables", or if they were reloadable, and if so, what load data. They wrote back saying they were perfectly safe with any normal load data.

    So, now (FINALLY :rolleyes: ) my question. If the WW with the huge flash hole are safe for reloading, why are the blank cases, with the huge flash holes dangerous? Are they really dangerous, or is this an old wives' tale (like deactivating primers with oil) that has been around for donkey's years, and every body "knows" it and repeats it, but there is no hard data to back it up?
  2. TranterUK

    TranterUK Guest

    Your first paragraph shows a good understanding of the dynamics of the detonation, right down to the movement of the primer in its pocket. Like the premature detonation of a cartridge in a blowback SMG, its not appreciated by most.

    Having such a thorough knowledge, I can only say to you just stick to tried and tested components. The pressures are immense, and components well tested. If you have a quantity of brass with bigger than normal flash holes, trash em! Blank or ball.
  3. mrkirker

    mrkirker New Member

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    I've noticed that the flash holes in my brass tend to enlarge as they near the end of their life cycle. I vote with Tranter - "Trash 'em!"
  4. Haligan

    Haligan Active Member

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    Alpo, as Tranter said, excellent explanation of the internal balistics that occur when firing a round.

    However,
    The Venturi effect discribes what happens with fluids, not gasses.
    Say for instence your using an eductor for foam for flammable liquid fires.
    The eductor has an inlet and an outlet for water to pass through. As the water passes over a small hole in the tube it "sucks out" the foam and mixes it with the proper amount of water. This is an example of The Venturi effect.

    http://en.wikipedia.org/wiki/Venturi_effect

    Hope that helps:D
  5. Bindernut

    Bindernut Well-Known Member

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    :) A venturi effect can happen with gas though too Haligan...all it requires is flow. Since gases are compressable as opposed to liquid the exact math involved is a little different, but still the same effect.

    Scientific theory aside....I've got a few of those Winchester NT cases with the large flash hole in .45ACP and haven't had any problems with them. I do keep them segregated from my standard brass and they're only used for light cast bullet loads.
    I think in a low-pressure situation they are fine.
    Now in a high-pressure load or cartridge I would not use any brass that has an enlarged flash hole.
  6. Popgunner

    Popgunner New Member

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    Enlarged flash holes will increase pressures dramaticaly.
  7. Oldeyes

    Oldeyes Member

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    The folks that develop sub-sonic rifle rounds often recommend boring out the flash holes to increase the effectiveness of primer ignition of the slower burning and typically lower pressure powders. They also recommend keeping the over bored flash hole case segregated to avoid potentially quite dangerous over pressure situations resulting from using those cases with standard faster powders and load data. Thus, in your case I would tend to err on the side of caution and not use the over bored flash hole cases in my normal reloading routine.
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