Thread: necking up 243 to 7mm08

Originally Posted by Bill999

What I can tell you is if you don’t size them to 30 cal then back down to 7mm approximately 3 of 10 blow primers and make you seriously reconsider how smart you thought you were beforehand.
Hard lesson but thankfully no injuries and just a bit of gas in the face


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Probably excessive headspace in action due to the steeper shoulder. Necking it straight to the required size isn't placing the shoulder in the right spot for correct headspacing so the head of the case isn't supported. That's why the step up to .308 or 7.62 is important, as when it's necked back down to 7mm it leaves a 'false neck' or a donut ring in the right place for headspacing. With that much shaking the neck size all about, annealing would be important I'd think. Especially on the first firing when the shoulder blows out to the chamber size.

Otherwise, it would be easier taking .308 cases straight to 7mm but it is safer to neck brass up as the wider pill dimension usually prevents chambering. I've had one .308 that swallowed a 7mm08 - wasn't very successful and the grouping sucked but I'm lead to believe that you shouldn't actually be able to do that?