While I haven't actually done any reloading for the .300 WSM, I would expect that overall length concerns would be at the top of my list. Will your magazine box let you seat the bullet out to where the bullet isn't pushed far backinto the case - limiting powder capacity and driving pressures up? How about the throat of the barrel? If the magazine box isn't an issue, is the throat long enough topermit seating out to where the base of the full diameter bullet is about even with the base of the neck? Twist isn't usuallythe limitingissue - a 1:12 will stabilize a 200 grain Matchking easily. A 1:10 (standard for the .300 Win Mag) will stabilize a 220 gr. Matchking.