It is clear it is a cracked neck. Check your cases better. Larry
Thanks Larry, but let me assure you I check my cases quite well, especially the ones I use in competiton. However, assuming I was lazy about prepping my special competition ammo (which I am not), don't you think that I would have detected a cracked neck after the fact while examining the cartridge in question and starting this thread, especially under high magnification? I assure you the case is NOT cracked in spite of what you think you see in the photograph.
I'll try to address some of the other numerous questions/guesses all at once.
The damage was not caused by firing; notice the bullet still in the case and the primer is intact.
Eliminate a ball type expander associated with sizing dies. I don't even own one. I also don't own any ceramic balls or steel BBs.
These special competition rounds were all loaded at the same time. I examine each piece of sized and cleaned brass as it rotates in the annealing pan, paying particular attention to the condition of the neck. A distorted case would surely have been seen.
The next step is to prime the cases, insert a final uniforming mandrel in the neck (not a ball type expanding mandrel) and I again handle them individually as I pour in the carefully weighed powder charge; another step which this dented case is very unlikely to pass. The charged cases get put into a loading block and then are examined under magnification with a strong light to check the charges. A dented case is likely to be detected during this step as well.
Next is bullet seating requiring individual handling twice, once as the case is put into the press and again when the cartridge is put into the storage box; two more chances to see/feel the dent. Plus, the way the case is dented, it would be VERY difficult to seat a bullet past the dent. I would have noticed that extra seating force.
I believe it is not possible for the case to be damaged this way during any of the above processes and highly unlikely for it to be undetected during these several steps before it got into the storage box.
So, after giving it additional thought, I have to believe the dent occured during chambering and I highly suspect a loose primer. Or it could have been something else, but from where? I re-examined the fired cases once again yesterday and each has a spent primer. The Lapua cases are relatively new with tight primer pockets and the load was far from "hot".
Bottom line: I'm chalking this up to one of life's mysteries and unless it happens again, I'm prepared to stop worrying about it. However, I'm afraid I've started a thread with a life of it's own.