[Gbro]

From loademwells question;

thanks! I bet that was the problem I was having. My mentor (who is an old school reloader) wasn't advised that I had neck sized die's. or didn't understand it yet. He told me "Always use lube on your casing" Even when I questioned him about the "lubeless dies" he replied; "Don't know nothing about them, but I'm sure it wouldn't hurt anything."
So all in all, I bet that was the prob. While letting the live round sit in the gun for a while, the lube fused inside of the chamber. Shooting it off must have jolted it free.
Lessons learned, Clean all finished rounds off before you load them up.

I was including my one time hard to open bolt experence and that the cause was overpressure and that over pressure was caused by the RCBS case lube cementing the bullet into the case.
I had a complete case separation and blown primer with that incident and when i tryed to pull the bullets on the other cartridges in that lot i pulled the bullet in two and it didn't come out of the case. I then put them through the press w/seating die treaded in a couple turns and each round broke free with a crack and a very hard lever pull.
Now for your incident.
Just how long had this cartridge been chambered in your rifle?
If over a good number of months for what ever your reason to do so is and the fact that you were neck sizing for what ever reason and the rounds were not cleaned of lubricant, then I can then see that you might have your answer that was basicly stumbled on to with my answer.
I would certainly do some feeding tests with the rest of the cartridges in that lot and pay attention to how they come out of your ammo packaging as that may give you a confermation that the lube is hadening.