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Neck turning, how often?

Realistically disregarding the humor once is enough unless neck thickening is effecting case neck tension , assuming the load is for the same Rifle !
However I could be totally full of shit .
 
I only do it once during the initial case prep from necking down brass. I have not came across having to do it twice for brass as of yet. I anneal, resize, trim, expander die, turn the necks, then resize again back to spec.

The only reason I do turn the necks is because 308 necked down to 260 wont chamber in my rifle. I don't for any other round though.