You said the neck expanded from .313 to .318 when you seated the bullet. Apparently your brass was longer than the end of the seater. When you lowered the press handle, the brass hit the end of the die and folded or rolled over. Depending on the seater die, some are designed to crimp the bullet, others not. Some crimp using a long cone so the mouth of the case is gradually pressed into the bullet. Lee collet dies work sort of like this. If you had this type, the case mouth would not expand. Other dies roll the case mouth into the bullet. Just a guess, I suspect that you have one of the "roll the case mouth into the bullet" kind and, because you screwed the seater "plenty" far, there was plenty of case mouth so as you seated the bullet it rolled over, contacted the bullet, and made a ring. Or something like that?
Assuming that you haven't already reconfigured your seater, sort through your brass and find the longest one. Set up your seater the way you always did and run that piece of brass into the die without a bullet. Pull it out and look at the case mouth, you should see a ring of brass. Suppose there had been a bullet in there. When the die rolled the case mouth into the bullet and then you pushed harder it would have pushed that "case mouth ring" down toward the primer and radially out even further and there would be your .318.
Back out your seater die so the case mouth doesn't hit the end of the die and then make the seater stem longer (screw it deeper into the die) in order to get the COAL you want. Ideally your die will have a lock ring with a set screw. If you don't have one like that, you can buy them from Brownells and lots of other places for a few bucks. Loktite that set screw and tighten it down then this won't happen again. That should sound like the voice of experience.