Severely Dented Cases….Secondary Explosion Effect?

[Nitroman]

And my guess as to how dented cases happen in the 7 rum; a mild load that does not develop pressure fast enough to seal the neck to the chamber but does develop enough to seal the case body to the chamber. The gas that has sneaked back past the neck collapses the shoulder of the case. and as you have stated there are many contributing variables such as brass hardness, etc that can help contribute to this happening. When the conditions are right-it happens. JMHO but its just a wild assed guess mostly.

This is exactly what is happening. If you had Pressure Trace you would be able to diagnose this right away.

 

[Pdvdh]

I've experienced this when loading for my 25 RUM with a very slow burn rate powder. If the powder charge is too low in weight, these indentations were experienced. I've read that the cause is low powder charges with slow burning powders, and that's the combination I was using the only time I've experienced this with my reloads. I started off with too low of a powder charge with a very slow burning powder - US869.

 

[texastrophy]

Had the exact dents on my 6.5 Creedmoor brass. Took out the bolt and noticed a miniscule, charred piece of lint in my chamber possibly left there from a patch. Not sure it was the cause, but it hasn't happened since I got it out.

 

[Kevin Thomas]

As others have already mentioned, it's caused by too light a load not fully sealing the chamber, and allowing gas to come back between case and chamber walls. Step up the powder charge a bit and this will completely disappear. Fairly common in very large capacity cases, especially with some of the slow burning powders. These propellants don't deal well with reduced charges, and this is merely one of the (nicer) ways they show their displeasure.

 

[307WYO]

Thanks to everyone for the help, input, and advice. I tried some Retumbo loads in the rifle and they are working great without any issues. I am going to try to bump up the charge with RL-25 to see where it settles out at. I will post the results as soon as i get them. Thanks to everyone again.

 

[chickm1]

There is not enough back pressure to cause the burn rate to produce the pressure needed to expand the case and seal it against the chamber walls. So, the expanding gas leaks between the case and the chamber, but can go no further and while it is continuing to expand, it dents the case inward. Go to a faster burning powder or a heavier bullet.