If your going to make .243 cases from .308s it should be done in three steps. From .308 Winchester, to 7-08 Remington, to .260 Remington and then to .243 Winchester. Bullet seating dies in 7-08 and .260 with the seater stem removed will work fine for the initial neck reduction, then a full length .243 sizer. What you'll end up with is cases with very thick necks, that should be inside reamed or outside neck-turned, and they will need to be annealed because of working the brass. The overly thick necks can cause severe pressure problems unless your rifle has a large neck/throat diameter.
Yup, I would step down in at least two steps. Three would probably be easier though if you can scrounge up the dies to do it.
Definitely will need to anneal the necks before starting and it wouldn't hurt to do it again afterwards either. All that necking down will work harden the brass.
You will need to turn the necks after you get em down to size too. They will be THICK.
I made a bunch of .243 from some NATO brass quite a few years ago. One of my first experiments with annealing and neck-turning.
I actually used a .22Hornet sizer die to do the intermediate step-down on most of the neck (it's what I had to work with).
Was a lot of work when .243 brass was easily available...I just did it out of curiosity. I sure wouldn't do it just because of cheap NATO brass. Once you figure in the time and effort, you have to consider if it's worth it. That's your call though.
Thanks folks. My SIL just brought me a 5 gallon bucket full of NATO brass that was free. I may just have to look around for some other dies. No sense in letting it go to waste. I already have the annealing gear and turning tools.
Maybe I'll just let it sit until the brass I have is worn out. I don't mind doing the initial work, in fact, I consider it theropy.