Re: .308 FL sizing die question.
l'd be VERY surprized if your RCBS sizer, or any other sizer, isn't reducing the necks sufficenty due to your minor truning, It would NOT surprise me if your expander is a thou or so over size. Mic the expander ball; it should be bullet diameter or a thou less, but not a bit larger than the bullets.
If it's too big, chuck the stem in an electric drill, spin it and use a bit of medium/fine grit (220 or 320) black carborundum paper to work it down to the right size. Don't sweat trying to make it glass slick, that really won't matter.