Powder Bridging??? Fiftydriver

Discussion in 'Reloading' started by robbor, Jul 19, 2005.

  1. robbor

    robbor Well-Known Member

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    Hey Fiftydriver, what exactly is powder bridging? I have noticed you mentioned it a few times. The only info I could find was about the powder getting stuck when it is poured into the case. And that doesnt sound like it.
     
  2. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    Robbor,

    Powder Bridging is basically the same thing you discribed but in the opposite direction under alot more pressure!!

    Basically, when X amount of powder is trying to pass through a certain diameter hole, it has a limit in how fast the powder can flow through this opening.

    With a very large case capacity round with a modern, sharp shoulder angle, this becomes a problem with smaller caliber cases. Now this depends on the caliber as well as the diameter of the case powder column and the total powder volume.

    What happens is when a round is ignited by the primer, the blast from the primer forces the powder in the case foward until it is stopped by the shoulder and seated bullet. Then it begins to ignite.

    As the pressure inside the case increases from the rear of the case forward, it increases the pressure on the powder. Powder bridging occurs when the pressure actually locks the powder granuals together because they simply can not pass though the neck of the case and pressure increases dramatically.

    In the case of my Allen Mags, the 257 and 6.5mm are the only two that so far have had this problem. The simple cure is to use Ball powder. This solves the problem simply because this powder shape can flow at MUCH higher rates through even smaller neck diameters then any stick powder.

    In the 270 and larger Allen Magnums, this has not been a problem at all but I still generally use the real slow burning ball powders simply because of their lower burn temps and they are much easier on the throat of the chamber as the powder is forced through.

    Hope this helps some.

    Kirby Allen(50)
     

  3. Eaglet

    Eaglet Well-Known Member

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    Kirby,
    Sorry amigo!
    If I had posted that question you wouldn't have had to explain it twice.
    I hope you don't mind my posting the answer you gave me. Just another angle that helps! Here it is:

    [ QUOTE ]
    Javier,

    Powder bridging is a situation that occurs when there is to much powder to pass though the shoulder/neck area of a case when a round is fired in a rifle.

    Basially, when the primer ignites, the force of the primer blast applies force to the rear of the powder column. In the case of a powder bridging situation, this force locks the powder if you will together so that it will allow itself to be driven through the case neck, locking the powder in the case.

    Well, when the powder ignites from the primer blast, pressure will build. Now if the powder bridge breaks loose and can be forced through the neck and out of the case while burning pressures will be relatively normal, BUT, if the powder bridges solid enough, pressure spike dramatically and will create an unsafe pressue curve.

    This is why Ball powders solve this problem, the flow very evenly and easily though the shoulder neck area no matter what amount of powder you have in the case.

    Hope this helps some!!

    Kirby
    Allen Precision Shooting

    [/ QUOTE ]
     
  4. 257speed

    257speed Well-Known Member

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    I had this problem in my 257 STW using RE-25. Switched to Ramshot Magnum and no more problem.
     
  5. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    257 Speed,

    How does the top velocity with Magnum compare to top loads with Rl-25?

    Just curious, they are supposed to be nearly identical in burn rate, just wondering if you found this to be true in your STW?

    Thanks,

    Kirby Allen(50)
     
  6. davewilson

    davewilson Well-Known Member

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    was wondering the same thing myself.
     
  7. ICANHITHIMMAN

    ICANHITHIMMAN Well-Known Member

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    I'm draging this out of the basement. I just learned something and I think others could benifit from it to.
     
  8. meatyrem

    meatyrem Well-Known Member

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    Can anyone give some examples of the slow ball powders?
     
  9. Boss Hoss

    Boss Hoss Well-Known Member

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    Last edited: Nov 7, 2011
  10. htduckman

    htduckman Active Member

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    Sure WC872.
    It's a military ball powder used to load 50BMG and 20MM Vulcan ammo.
    Burn rate is very similar to AA8700.
    You can use AA8700 data.

    I'm loading it in 264 Win Mag and 7 STW.
     
  11. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Sorry Boss,

    short of doing some testing in the lab (with the appropriate gear to actually measure what's going on here), it'd be nothing more than a guess on my part. No dog in this fight, and not much of an opinion on it one way or the other. Frankly, it doesn't apply to any of the cartridges I routinely deal with.
     
  12. Boss Hoss

    Boss Hoss Well-Known Member

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    Last edited: Nov 7, 2011
  13. royinidaho

    royinidaho Writers Guild

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    Stick any of the stick powder in amounts to get a decent velocity other than 7828ssc in a 270 AM and you will learn all about what we are calling powder bridging.

    Oh, and don't even think of using any thing heavier than a 150 bullet.

    It wasn't a planned pressure test of the ol Rem 721, but she held together . . . several times.:rolleyes:

    Case life was something less that 1 firing.......
     
  14. gunpower

    gunpower Well-Known Member

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    I would like to know how you would know that this is happening in your
    cartridge case? I have been reloading for 40 years and this is the first that I have heard of powder bridging in side of a case. I have use many types of powder over the years and have never had any problems as to what you are saying, that I know of!!! How do you tell, how do you find out this is happening??
    Would the use of a mag. primer solve the problem??