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First thing I thought when I heard preprimed...did you prime it yourself? If not it's possible it was never sized properly, allowing setback. I've had bullets push right into incorrectly sized brass.

Did you make any modifications to the hammer?...like using a shell holder instead of the collet? The primer not setting off the powder is not too hard to believe, the force from the unrestrained primer just blows it out of the primer pocket and not enough of the flash makes it into the case to ignite powder. Glad everything turned out ok, I've watched guys really beat on those things and cringe every time :-)