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7RUM case denting when firing (pic)

 
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Old 11-08-2008, 02:31 PM
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7RUM case denting when firing (pic)

Loaded some Berger 168's over 94gr Retumbo. Case necks are partially sized and bullets were seated long to touch the lands. Base of bullet is at neck/shoulder junction inside the case.

Below is a pic of the result.

Never seen this before and I am assuming it is due to the small amount of bearing surface held by the neck not allowing the case to expand enough to seal the chamber before the bearing surface exits the case and gas is allowed to flow around the boat-tail and back into the chamber.

I base the above conclusion on not only appearance, but also measurement. Outside case neck diameter on loaded round prior to firing was 0.314. Outside case neck after firing is now 0.313. Internal neck diameter before seating bullet was 0.282. Internal neck diameter after firing is 0.282. Case did not grow in length at all, nor did body diameter change.

You can see in the pic that soot is evident on the case body along its entire taper. There is a definite ring at the case web where the soot no longer traveled rearward.

Am I on track with this? I have ideas for remedies, but wanted to get input from others as well.

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Old 11-08-2008, 05:06 PM
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Re: 7RUM case denting when firing (pic)

Hope all is ok with you and your rifle, the first thing that comes to mind for me is there any thing that is in your chamber. I can see the neck and shoulder have blown out making it look like it did not have a good fit in the chamber. Was the bolt hard to close? Any other out of the ordnary feel with the rifle. Was this the first time you shot the rifle? I would not fire any more rounds until you figure this out. Hope you can find out what is causing this. Oldfamily.
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Old 11-08-2008, 05:20 PM
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Re: 7RUM case denting when firing (pic)

you're shooting too low of a charge. there's not enough pressure to seal it around the neck and it's escaping back causing the dents. i'd also be concerned with the clearance of a loaded round. sounds too tight to me.
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Old 11-08-2008, 06:38 PM
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Re: 7RUM case denting when firing (pic)

oldfamily: I'm ok and nothing out of the ordinary happened from my perspective when I pulled the trigger. Nothing in the chamber, nothing wrong with the rifle, been shooting it for over a year. Contrary to the way it might appear in the pic, the case is not blown out, but rather did not expand enough to seal the chamber.

davewilson: Low internal case pressure is what I've concluded as well. What clearance are you referring to when you say, "i'd also be concerned with the clearance of a loaded round. sounds too tight to me"?

Thanks for the input fellas.
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Old 11-08-2008, 07:11 PM
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Re: 7RUM case denting when firing (pic)

He is referring to the clearance between the neck of the case and the neck of the chamber. I like to see atleast .002". I have seen the same thing and it was caused by to low pressure loads. Either increase the load or go to a faster burning powder.
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Old 11-08-2008, 07:43 PM
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Re: 7RUM case denting when firing (pic)

From what I am reading, then, the case is not obdurating to the chamber wall?

This is happening because the charge is not high enough to raise internal pressure and cause the brass flow and obduration?
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Old 11-08-2008, 07:44 PM
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Re: 7RUM case denting when firing (pic)

6mm: If the case neck clearance to the chamber was too tight, wouldn't that result in less gas being able to escape around it and also less pressure required for a proper seal? If I'm not thinking in the right direction here, please help.

Interesting ideas about faster powders and/or increasing the charge. After posting this topic I tested the same setup with increasing charges up to 98gr of Retumbo. They all did the same thing without any one more or less severe than the one pictured above. What is interesting is that this does not occur with the same powder charges when using deeper seated bullets. Using Hornady 162gr SST and SMK 175's, which seat deeper into the case neck than the VLD's, the problem is non-existent.

Could be that a faster powder would develop enough initial pressure to expand the case properly. I guess that's the next step to take if I want to seat VLD's at the lands with this barrel.
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