From what I remember when forming cases from another
You will be able to make 243 cases but they might be short or long depending on how everything works out. Your necks may also be very thick and need to be turned. Check the specs for a 243 after you form one and check where you ended up. I also remember that the forming process will get you close. You will then load up a light load and fireform the case to the actual chamber of your rifle. Good luck and let us know how things turn out. Be careful if you are using military 308 brass as it is thicker than standard 308 brass.