What is “powder bridging”?

Discussion in 'Reloading' started by Calvin45, Jul 23, 2019.


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  1. Calvin45

    Calvin45 Well-Known Member

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    i’ve encountered this term a bunch and have looked into it but do not feel 100 percent certain I understand what it is. I know it’s a concern with stick powder in massively overbore cases. That’s about it.
     
  2. Mike 338

    Mike 338 Well-Known Member

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    Not sure about in the case but powder in the hopper when reloading can bridge, or as you continue to use up powder as gravity drops it lower in the hopper, you get a clump of sticky powder somewhere up high in the hopper and and that bridge of sticky powder prevents gravity from feeding down to the lower portion of the hopper. I imagine a centerfire case is just a smaller hopper if you stand it upright.

    I've disassembled cartridges and found sticky clumps of powder that I had to shake really hard to get all the powder to come out of the case. That's probably what they're referring to. I suppose a hard ball of stuck powder can significantly affect even burn characteristics.
     
  3. elf

    elf Well-Known Member

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    You nailed it Mike
     
  4. johnnyk

    johnnyk Well-Known Member

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    To add to the clarification here is an explanation from Kirby Allen;

    I think I read on here where "RoyinIdaho" experienced this with his .270AllenMag but I didn't search for it.
     
  5. royinidaho

    royinidaho Well-Known Member

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    Yep that was me...

    The above describes it.

    Envision a super over bore cartridge such as the 270 AM, which Kirby said to use ONLY ball powder. He musta known something!

    Load ‘r with a little under 100 gr of a big kernel powder such as RL-50 and a 170 grain bullet. Yep, a good way to proof test your circa 1950s Rem 721 action.

    Powder bridging = a plugged toilet. Only quicker!

    Get that image outta your head!
     
  6. azsugarbear

    azsugarbear Well-Known Member

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    Powder bridging most often occurs in cartridges that have these three things in common: 1) stick powder; 2) "overbore" cartridges (as in powder charge to bore diameter) and finally; 3) usually an improved case. Most cases are based off of military designs that have gently sloping shoulders for better feeding and extraction of a round. When a case is "improved" it usually involves moving the shoulder forward and giving the shoulder a more acute angle. This improvement allows more powder capacity in the case. The improved shoulder cuts down on the amount of stretching that occurs in the neck/shoulder area, thus improving case life. Some also feel that it also helps mitigate throat erosion. While I cannot address the idea of less throat erosion, I can opine as to the case life of improved cartridge brass. I used to own a 277 Allen Mag. It was based on the 338 Lapua case and burned over 100 grains while propelling a 195 gr. bullet at about 3,150 fps. I had more than 12 firings on nearly all my Lapua brass before any signs of thinning brass became apparent in the wall close to the case head.

    The downside of this type of cartridge design is that it was prone to powder bridging. Due to the acute angle of the shoulder, along with a large amount of powder trying to get out that .277 bore, bridging became a potential nightmare. Pressures could/would spike quickly and become very dangerous. You couldn't predict when it would occur as it was not a consistent. But the potential for extreme spiking was enough to convince a shooter to not go anywhere near that cliff. I ran only US 869 ball powder in mine.
     
    Mike 338 and Calvin45 like this.
  7. Mike 338

    Mike 338 Well-Known Member

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    I'll be darned. Somebody besides me uses ball powder on occasion!
     
  8. johnnyk

    johnnyk Well-Known Member

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    That's all I have used (US-869) in my .270 AM. I would like to try Retumbo or some of the slowest Vihtavuori's but I feel like it's Russian roulette. :)
     
  9. tdot

    tdot Active Member

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    So maybe this is a dumb question... I'd read about powder bridging before and understood it to affect the loading process. But from what I'm reading here, it also effects the ignition of the powder? Is this correct?

    Does this assume that not all the powder is being burned in the case?
     
  10. azsugarbear

    azsugarbear Well-Known Member

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    You are correct in that 'powder bridging' can occur both when reloading and also when firing overbore cartridges. With reloading, it is no big deal (unless you are using a progressive reloader). Just use a funnel with a larger mouth, or pour the powder a litter slower and the problem is solved. When it happens while firing, it becomes a big and dangerous deal. That's probably why there were so many responses regarding powder bridging when firing.

    Powder bridging when firing still burns all the powder, but creates a huge spike in chamber pressure.
     
  11. Lefty7mmstw

    Lefty7mmstw Well-Known Member

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    In actual use, you have a hard time finding a stick powder in sub/ 7mm bores that is suitable for the extreme overbore cartridges like the 6.5/ 270 rum or the Allen mags... Retumbo is already a bit fast burning in something like a 7rum (I was having pressure issues before I got to 90% fill).
    Going more overbore and trying to use smaller kernel stick (that might actually flow under pressure down a 6.5 or 25 cal bore) just isn't going to happen (no one really makes the powder) so you either rick burn issues (possible gun damage/injury or at the least a poor performing load) from bridging or you put up with ball powder and sometimes having to adjust load/ velocity for temperature.
    In my 7rum (and 7stw) I am lucky; I've never had bridging issues with a 7mm bore and 5010/ 50BMG or rl50 (rl50 DOES act happier in a 300rum pushing 225's). I'm probably on the edge of this happening though, with the 270AM having the problem.