I have built both .25 WSM's and a 6.5 WSM. The former turned out to be too much of a good thing - at least with powders currently available. The 6.5 WSM wasn't too bad - it gave velocities a plateau above the .264 Winchester Mag and produced standard deviations within an acceptable variation - using RL25. The 6MM WSM is going to be a barrel eating machine; and I would be surprised if it produced speeds much higher than the 6MM-284, which I consider to be the biggest case practical for this bore diameter.