Re: Wildcat brass (necking down)
I'd think the best way, as long as you weren't playing too much with overall length, would be to get a neck sizing die that uses bushings and get a few in between .30 and .243. I'd think a step with a 7mm then 6.5 mm equivalent bushing before finally going down to .243 should be easily doable. Make sure you anneal your cases first and use a good lubricant.
Another consideration in going down that far is that the case may form a "doughnut" of brass right where the neck meets the shoulder. You may have to do a round or two of neck turning in between one or all sizing steps as well.
I'd consider getting a custom set of dies made to your dimensions. This I haven't done but I would assume there's someone out there who can cut them from a dimension drawing.