Necking 6BR brass up to 30 cal and above is not difficult. Do it in stages (I go 7mm then 30 cal) and use plenty of case lube.
I have lost a few case necking up but then again I made a lot of 30BR brass up from worn out 6BR cases.
If it did it again I would go straight to 338BR and try something heavy (300 grains) and something light (something by Barnes or Le High).
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