I'll posting a reply made by John Barsness over on 24 hour campfire. The topic is about erratic pressures with the .243.
"The .243 does stretch and thicken necks in some rifles. The first .243 I owned did, and since I was beginning handloader some strange things happened before I figured it out.
The other problem with the round is that throats can erode more quickly than with some other, similar rounds. The best guess is that this is also because of the shoulder angle and short neck, combined with the relatively small bore. Contrary to what our forefathers believed, an eroded throat actually increases pressures, for a couple of reasons, at least at first. There have been some .243's with worn throats that blew up in pressure labs, with everything on record, so this isn't a figment of the imagination.
But with normal care and some knowledge of the possible probems, a .243 will do fine work for a long time. Certainly a couple of mine have. "
The rifle my son had the high pressure with is a .243 Savage 110. He used the X die instead of trimming and had trimmed the brass as per the instructions before using the die to control the brass length. My .243 brass "grows" and I trim it after each resizing. The .243 is not a cartridge I'd use a X die for. Your experiences may vary.