I have done it both ways (water quenched and air cooled) and I can't really see a difference on .308 and longer cases. If I was doing short cases, 300 BO or Hornet cases, I would water quench to keep the heat from reaching the head. What kind of annealer are you using?
I have air cooled and I have water quenched. I have read that both methods are the "best". From the small amount of metallurgy I have picked up trying to answer the same question I now air cool all brass I anneal. Not saying that is the "correct" answer, just what I do. I also agree with AJ that short cases care must be taken to prevent softening the head. I use an Annealeez annealer.
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