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10mm reloading case failure. What could cause this?

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I agree that fast powders are NOT what the 10mm was designed for, but they are perfectly safe and this in no way looks like a double charge.
That is NOT from a magnum primer. That is NOT from a slightly low COL (and, no matter what the COL is, you still start at the start load).
Reloading mistakes generally show up around the extractor groove: Guppy belly, blown out at the web, blown out through extractor groove, or bulged case head.
All I could say is that based only on my experiences, that is a case that had an inclusion that weakened the brass at that point.
Here is a picture of an overcharge:
KaBoom PICT0335.jpg
 

spt_1955

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I'm having a hard time attributing this to anything other than a failure of the brass. That is a very clean split. Any pressure spike would not split a case cleanly with no bulging or tearing of the brass. An overpressure charge would not split a case in the middle where it is supported by the chamber walls. It would blow out the back of the case and the primer would not look as rounded as that one does. It would either be flatted against the breech face or pierced. (Unfortunately I know what a double charge does)
 

Arto

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You'd think it would split axially if pressure were too high or material was bad, since hoop stress is 2x axial stress (if it were a free cylinder)
how is the case length vs. headspace? heaviness of crimp? (i.e., is there axial clearance for the crimped-in-bullet to stretch the case?
 
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OP what's the primer look like ? That casing looks to me like a classic case separation from excessive headspace. I'm not seeing any bulging near the base of the casing that's usually present with over-pressuring ?
 
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