I see this is a pretty old thread but for someone new reading this, I thought it might do some good.
I have made lots of .243 brass out of .308 and once you've ironed out the quirks, it is pretty straightforward.
Trouble you might have is if you get any lube on the outside of the neck or shoulder area. Then, you can get shoulder wrinkles or even serious lube dents-keep the lube away from neck and shoulder area and no troubles.
Go to the Natural Food Store and get a jar of lanolin. It works way better than any sizing lube they sell-unless you like sticking cases in dies.
Put the lube on the outside of the .308 case to be sized and none on neck or shoulder area.
Take a small rifle brush and run it inside the neck and clean out the crud and carbon as best as you can.
Take a small amount of lanolin on your finger or put some on a paper towel and just get a little ring of it inside the case mouth so your expander button doesn't stick.
Wipe the outside of the neck and shoulder area clean.
Now size in your .243 small base die set and-bingo. Good idea to fireform and then see if a .243 bullet fits inside easily.
If it does, no reaming necessary. Trim and load-or ream if necessary.
If you have all the stuff to ream and trim already, then no big deal. You may have to buy a neck reamer if you don't have one already but that's about all it takes.
At today's prices for brass, it is worth it to go the .308 resize route since you can usually pick that brass up at gun shows pretty cheap.
