I have one in my MEGA .308. I used a standard milspec receiver extension with the JP captured spring but still had to fill in a little space to get the spring to contact the bolt carrier when assembled. JP said to drop a quarter down the receiver extension to fill in the extra space if needed. This worked well for me and fixed the accuracy issues I was having. I only had about .08" of play between the bolt carrier and JP captured buffer and it was doubling my group sizes.
For the SR-25 / LMT MWS, I would cut the included spacer to fit those extra long extensions.