Reforming 308 to 243 Win and 260 Rem.
I've never had trouble, even neck diameter works in my guns but you'll have to check your own chamber to be certain.
If the inside diameter of the neck is too small, seating the bullet makes the neck too fat for the chamber.
It may be an issue if your chamber is tight at that location, but I've never had a problem myself.
Be sure to check the first few before you do 1000. Use a neck reamer if necessary.
Sometimes 308 brass comes out a little bit short on the neck but not such a big deal. You still get plenty of neck to hold the bullet.
Just be sure to keep reformed brass separate, the headstamp still says 308 or 7.62. You don't want someone to get confused and try it in the wrong gun.
Best of luck. It works. Go slow. Use plenty of lube. I reform in a standard sizing die without the expander first, then go back again with the expander properly adjusted. It's just easier to do it in steps.