If you neck up by pushing in an expander mandrel, keep an eye on run out. The wildcat's I've done this way came out straighter with a square case mouth. It might depend on your specific tooling, but I've had best luck with doing a few small steps vs one big one. A nice taper, polished finish and lube help.
If you are going to expand the cases by firing them with pistol powder and filler then it probably doesn't matter.
308 brass is cheap to experiment on--let us know what you come up with!
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