Berger bullets have a pressure ring on the 243/6mm bullets i use. Its different from lot to lot. From .0004" to .0008" larger than the main bullet bearing surface diameter of .243" Does not effect neck tension, using bushing dies, no expander. If your .308" bullets are undersize, as it seems they are, it may be a problem for accuracy, but expanders in the FL sizing die are at least .0015" or more smaller than bullet diameter. Add a little springback of the brass, and neck tension should be fine.
Brass with thin neck walls or a FL die not sizing small enough, would seem to be the main problem.