Re: .300 win mag vs. .300 WSM once again...
Lots of good comment here, I have several short mags, and my 300's have in fact been load picky. Having said that, my others (270 & 7) have not. Comparing all to my 300 WM's.
None of my WSM's shoot discernably faster than a WM apples to apples as best as you can do that with these cartridges, which I guess are more like apples and cucumbers--my meaning is that if you keep pressure equal, bulet weights equal, powder charges may vary slightly, but not faster per se.
Many advocate that the shorter action is stiffer and therfore more prone to accuracy, and I think that can probably be proven empirically. Going to a long action and using a WSM case seems counterproductive to me, if you go long action mag, and don't want a belt, look at Dakota's cartridges, the RUM's, the Lapua and probably a ton of other stuff I don't know of offhand.
I think the home for the WSM is light to mid weight for caliber pills, and lighter flatter shooting type rigs. Now there are a ton of guys shooting the 7 WSM in long range and lighting it up.......but WSM vs WM, to me is easy, how heavy a bullet you want to shoot? If it's over 180 as others have said, go the WM and don't look back.