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.