Large Denting of Fired Cases

I built a 6.5CM off of a varmint action after owning at LRP in 6.5CM. I noticed that some of the cases were being dented after firing. Ive seen this sort of denting in a semi-auto rifle but never in a bolt action. I have tried opening the bolt slowly thinking maybe ejecting the sent cases slower would lessen the problem but this is not the case. The rifle is still denting about 1 in 5 cases. I have had no issues with my LRP 6.5 and the chamber is clean. Does anyone have any ideas what causes this marking and what would be the fix for it?
Jesse

http://img.photobucket.com/albums/v2...v/IMAG0828.jpg

That is NOT a dent... that is case collapse. It is a phenomenon caused by a low pressure load that does not seal the neck and thus gas escapes around to the case and the pressure is trapped between the case and the chamber. Notice the soot on the neck/shoulder area, that is a sign of either not enough powder, or too slow of powder.

Fred beat me to it! :p

Sticky?

I was going to mention / suggest that but I thought when that happened to me it was due to my stoopidity or negligence...... Or both.

I didn't know it was a common occurance.

Good call Mr Sharpshooter....

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Originally Posted by sharpshooter

that is case collapse. It is a phenomenon caused by a low pressure load that does not seal the neck....

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Would this phenomenon be possible with in listed load data of a powder and a standard length barrel? The lowest loads that i was starting out were clocking in at 2688 FPS for a 140 Gr A-Max, most topping out around or above 2850 FPS before getting pressure signs with common 6.5CM powders (H4350, Hybrid 100V, RE17, 4831sc).

What exactly was the load? powder primer and bullet? I've had the same results with 4831 SC.

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Originally Posted by sharpshooter

What exactly was the load? powder primer and bullet? I've had the same results with 4831 SC.

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I cant say i know what the exact load was. This rifle is in the hands of the better half for its load development. I can say it has happened with 4831sc as well as with Hybrid 100V at what charges i can not say. The rest of the equation would be 140 gr A-Max, Hornady 6.5 CM brass, Federal Large Match Primers (GM210M). It sounds as if you are suggesting a faster burning powder. I have some Varget laying around I could throw a few loads together with.

I think I would give H4350 a go. Hornady picked that powder for their factory 6.5 CM ammunition for a reason.

Question: If (for whatever reason)the necks were really work hardened and not annealed, could that keep the brass from expanding enough to seal the neck against the chamber walls?
In other words, would annealing stop (or perhaps lessen the possibility of) this?

The brass would have little to do with it, it's a situation that has more to do with the pressure curve. The peak pressure is occurring too late for the neck to seal. There are several factors that contribute to the pressurecurve, ignition from the primer, burn rate of the powder, and the length of the barrel.

This happen to me about 31/2 years ago shooting a Finnish M39 (7.62x54R). The Speer #11 manual on page 52 fas a picture similar to yours. While the above are good answers, also check for excessive freebore. Are you crimping your loads? Is the first shot always dented or just the second and follow on?

Just to add to my post above. I was using pulled milsurp bullets. The original "factory crimp" had distorted the bullet shape enough that the case could not get a good seal in the chamber when fired. Powder was not an issues because the original powder from the original steel case was used in the new brass case. The issue was solved by using a firmer crimp and preventing bullet setback.

Seems strange that every case looks dented in same place, if it was pressure related youd think they would be more random.

Is chamber clean??? any debris???