Flattened primers before shooting?

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Primers need to be fully seated to set the anvil into the primer pellet, and this normally means below flush. I seat all my primers .004" below flush with the base of the case.
+1. Unless there's bunch of crud at the bottom of primer pocket, you should be able to seat primers below flush. Only exception is trying to seat slightly larger diameter metric primers (Fiocchi, PMC, Tula, etc.) into tighter once-fired brass.

One benefit of using slightly larger diameter metric primers is extending the life of tired brass with enlarged primer pockets. ;):D

I would be more concerned of the possibility of crushing the primer anvil to fast into the primer solution.
I wouldn't at .004" to .008" below flush. We tested this in this thread - https://www.thehighroad.org/index.p...ts-your-experience.630512/page-4#post-7810685

"Here are some more primer seating depth and primer cup crush/flattening data for us to consider. I seated the following primers to the typical .004" below flush and .008" below flush that flattened the primer cups. The primers were hand primed using Lee XR priming tool with two thumbs and primer pockets were cleaned only without any modifications. (Yes, you can seat a primer to .010" below flush with the XR without the worry of handle breaking due to the heavier duty handle). Although subjective and personal, I will use a 10 point scale for "felt pressure" used to seat the primers with Winchester SP being 5.

Top row in PMC cases:
Tula SP - Firm seating pressure to .004" (8) and really firm pressure to .008" (10)
PMC SP - Firm seating pressure to .004" (7) and really firm pressure to .008" (10)

Bottom row:
Winchester SP - Moderate pressure (can seat with one thumb) to .004" (5) and firm pressure to .008" (7)
CCI SR (in Speer cases) - Moderate pressure to .004" (5) and firm pressure to .008" (7)

(In comparison, Magtech SP primer takes lighter pressure (4) to seat to .004")"

Summary - All primers seated to .004" and .008" below flush fired without issues. So feel free to firmly seat those primers.

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I concur with earlier posters - probably tried to prime crimped primer pockets. I prime with an RCBS hand primer - it works perfectly unless you try to prime a case that hasn't had enough crimp removed. The primer "crunches" into the pocket and flattened. All of the cases I've done it to are marked with a X on the bottom with a Sharpie and are in my "load last" pile. The X is to remind me to further decrimp them before loading again.
 
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