I am by no means a 6.5 WSSM expert, there are other people on this forum with much more experience with this wildcat than me. I will provide what I can on the subject though and appoligize up front if this is too much for a 25 WSSM thread.
Mine is built on a Savage Model 16 short action, originally a 7MM WSM
. I had Pacnor make a prefit Savage large shank, polygonal rifled, super match barrel and they chambered in 6.5 WSSM. I installed it myself using 25 WSSM head space gauges.
This version of the 6.5 WSSM is based off the 25 WSSM case so the Redding 25 WSSM bushing die can be used with 6.5MM bushings to size the cases. (I already had the dies from the 25 WSSM barrel.) The necks on the Winchester WSSM cases are really thick from the factory so I always turn them and then anneal them before using them. The Winchester brass seems a bit hard to me.
The 25 WSSM Redding seating die can be used to seat the 6.5 mm bullets. Works fine with the 140 Berger VLD and 140 AMAX.
I had mine made with an 8 twist so that it would be good for the 140 grain bullets.
So far I have only shot it to 200 yards and am still in load development. I have been shooting the 140 AMAX and 140 Berger VLDs and it is doing pretty well at 200.
I started a 260 REM project at the same time. They are almost equivalent in performance to the point where they work with almost the exact same powder and load.
I have been considering having it re-chambered to 6.5 WSM when the throat is shot out ...
As for long range performance of the 6.5 WSSM, I will leave that to the other guys with 6.5 WSSM to comment on.