I don't want to sidetrack the thread as flairing bottle neck rifle cases for jacketed bullets should be a topic for a different thread.
All of my straight wall rifle die sets have a expander/flare die from the factory.
Most bottleneck rifle sets I have seen do not.
My mental error, I was equating rifle=bottleneck since the OP was loading .243
Yep, a straight wall rifle round (which I wasn't thinking about) I imagine would probably have to be flaired just like a 9mm, .357 etc.
Lymans site recommends M dies for lead or flat based bullets in rifle rounds so you could flair bottleneck rounds for jacketed bullets if there was a need,
I have never had a need to.
Others may. I don't want the OP thinking he doesn't have something he needs to load .243. (I have never loaded .243 but I would guess it loads about the same procedure wise as .223)
Thousands and thousands of .223 and never flaired one....
I would imagine there have been millions of rounds of .223/5.56 loaded without being flaired, so is flairing them wrong, no, is it necessary, no.
But of course I have done other things wrong thousands of times as well.
No flair die in 2 die sets, or for example my 3 die Lee .223 set.
Resize/deprime/neck expand, bullet seat, crimp (if needed/desired)