Re: 300 wsm
Yep, if I were to build a 300WSM I'd be building it off of a long action as well. But the allure is to have a magnum cartridge in a short action so having it in a long action sort of defeats the purpose. Now on the other side of that argument, you can't harness the full potential of the 300WSM on a short action. But countering that, if you're going to use a long action why not just get it in 300WM because it typically surpasses the WSM with the 180Grain+ bullets. So a 300WSM is more of a "I want THAT caliber in my rifle" than the typical "THAT caliber suits my rifle/needs better" type thing.
UNLESS, you plan to only use the "smaller" bullets (less than 180grns), in which case the 300WSM definatly makes more sense because you use much less powder compared to the 300WM.
Hope this helps,
-Dylan