I'll go one farther, buy a box of new 243 ammo and test it. Even new brass has to be resized, so you wanna make sure your dies are setup properly as well. It only take a couple thousandths of extra length to screw it all up.

You DID screw the barrel down onto the GO guage, correct, and then tested with both GO and NO-GO guage? Also, on occasion, I have had to remove the extractor from the bolt head, to get proper headspace.