What you described is exactly what the signs of case head separation are. The number one clue is the bright ring forming around the case just forward of the head, and you proved it with the paper clip.
It is difficult to say if the brass was defective since you have never used any others under the same conditions. If it was quality brass it probably isn't defective, some calibers are just harder on it than others. It would be a good idea to replace it though, if a round ruptures in the rifle it can send hot gas and debris in you face, gum up the action and the front part of the case may have to be dug out of the chamber if it completely seperates.
If your barrel had a defect it would show up on all the cases and they would have probably shown up with the first firing and would show where the case was inserted in a different way each time fired.