The JP buffer is designed to work in a carbine RE for an AR15 pattern AR. To use it (or any carbine length buffer) in a 308 AR, you must use an A5 length RE. The carrier of 308 ARs is longer than the carrier used in AR15 pattern ARs.

If you want to stay with a carbine length RE on your 308, you must use the shorter 308 carbine buffer. I don't recommend doing so because the spring coils start getting pretty crowded when the carrier is to the rear under full recoil.