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Powder Bridgeing

 
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  #1  
Old 03-06-2006, 06:50 PM
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Powder Bridgeing

Hi i have no idea what powder bridging is can someone explain it to me
Thanks
DR B
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Old 03-06-2006, 07:48 PM
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Re: Powder Bridgeing

You know when you are pouring powder from the pan into the funnel and sometimes it clogs in the funnel and doesn't drop into the case. That is bridgeing.

What Kirby is talking about is when you have a very large cartridge case and a small idameter neck, the primer explosion will shove the power up into the neck and cause it to jam up and not be able to get out into the barrel. This causes an immediate pressure spike that is not good. This is more likely to happen with coarse stick powder than with ball powder which flows more smoothly.


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Old 03-06-2006, 09:02 PM
 
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Re: Powder Bridgeing

What cartridges would this happen in and what powder?
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Old 03-06-2006, 10:12 PM
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Re: Powder Bridgeing

Yeh, what Bob said.

Generally this will start appearing in rounds such as the 6mm-06 AI or 257 STW or something like that. The sharper the shoulder the more this will tend to happen. It is not common in the 257 STW but I have seen it in three of the rifles I have built for customers and my personal rifle slightly.

My 257 Allen Mag is terrible for this. You simply can not shoot any loads with any stick powders, every one will bridge and cause severe pressure spikes. If you use ball powders problem is solved totally.

Generally neck diameter needs to be 6.5mm or less. My 270 am does not seem to have a problem as the neck diameter is large enough to allow the stock powders to pass well enough to prevent bridging.

The worst powders are IMR with their very long granulars. The large Hodgdons are next I would say with the RL line being third with their slightly finer granular size.

The new short cut powders help alot and the newer ball powders are a god sent for my AMs, espeically the 257 AM and AX as well as the 6.5mm AM and AX.

There is really nothing you can do to prevent this in a case where it occurs. I have noticed that heavy for caliber bullets tends to help limit this to some degree. Probably because it causes a better in case burn then the lighter bullets.

Like I said, its not common in the 257 STW but it does happen enough to be aware of it.

Good Shooting!!

Kirby Allen(50)
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  #5  
Old 03-07-2006, 12:16 AM
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Re: Powder Bridgeing

What powders are the best choice for the 257 stw. I belive David Young recomends Ramshot Magnum.
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  #6  
Old 03-07-2006, 12:56 AM
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Re: Powder Bridgeing

[ QUOTE ]
There is really nothing you can do to prevent this in a case where it occurs.

[/ QUOTE ]

There is forward ignition as tried by Rocky Gibbs but it seems like it would be a fairly tedious undertaking.

For those who don't know what forward ignition is, it is where a long tube is threaded into the primer flash hole so the powder ignition occurs at the shoulder end of the case and burns back toward the primer. The theory is to have only expanding gasses flow down the barrel behind the bullet instead of burning powder kernels. I beleive this is done with some artillery shells as well.
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  #7  
Old 03-07-2006, 10:58 AM
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Re: Powder Bridgeing

Kirby,

I use the IMR stick powders in my .257 Weatherby. Is this "bridging" something I should be concerned about with that cartridge?

Thanks.
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