Much like the death of Mark Twain, the fears of dangerously thick necks from necking down are greatly exagerated, usually by people who have not done it. Obviously you have to measure the necks of loaded rounds to insure they are not oversize but it is VERY infrequent when making .243 from .308/7.62. Necking down adds little to the final thickness and our factory chambers are usually much to loose for that to matter at all.
However, if you use much longer brass as a donor, such as .270 or 30-06, you WILL have thick necks that need outside turning. If so, don't bother to use an inside reamer, it will only make the necks thinner without any reguard to concentricity, turn the outside to a consistant thickness. And using a file trim die makes cutting longer cases to the right length fast and easy.
I make a lot of my small cases from bulk .30-06 (making .22-250, .243, 6 mm International, 257 R, .308, etc.) specifically so I CAN turn the necks to be concentric and snug fitting in my chambers.
If all someone wants to do is blow holes in targets or game it's hardly "worth it" to form such cases. But, if you want those holes to be close together, it surely can be worth it to make custom fitted brass for loose factory chambers.
In your case (pun) stepping down .308 stuff in a single step with only a .243 FL die works but it's easier to have an intermediate step. Using either a 7-08 or .260 FL die, or both, (no expander buttons are needed) to step them down gently works great.
Much of the GI brass (7.62) is slightly thicker in the walls so you cannot willy-nilly mix the GI cases with .243 cases without care. As with any other component change, your loads need to be developed with the new cases as a base.
Using the expander button in the .243 FL die takes pretty good care of the dreaded doughnut because it expands the extra thickness to the outside before neck turning, if that's needed at all, so it's gone.
It is a good thing to properly anneal the finished necks to prevent premature splitting.
