The 300 WSM is an all around great cartridge. It has surpassed the 300 Winchester Magnum in popularity (according the ammo manufacturers) and has many advantages over it's cousin the 300 WM.
It is built off of the belt-less .404 Jeffery's case which is the first advantage. It has a 35º shoulder which improves the cartridge's efficiency. This is the primary reason why it can equal the 300 WM velocities with 10% less powder. In theory it has about 2 ft pounds less free recoil than the 300 WM in a average weight rifle (all things being equal). Regardless, 24-26 ft pounds of recoil is noticeable to most and uncomfortable for some. The woes of magnum cartridges pushing medium to large bullets.
The cartridge is a short action so there are two advantages. A shorter powder column has been proven to produce better accuracy (example: benchrest cases) and the short actions used on the rifles are stiffer and don't flex - another accuracy gain
The cartridge is best suited for 165-168 grain bullets (although its shoots all of the standard 30 caliber bullets quite well). You can enjoy about a 70-100 FPS increase over the 300 WM when using the bullet weights above.
Game has been harassed by 30 caliber magnums for years and are certainly overpowered for all big game in the lower 48. With that said, in the right hands, the 300 WSM is capable of taking all North American big game at extended ranges and has become one of my favorite cartridges. It looks different, it is very efficient, reloads well, shoots flat and strikes game as if hit by a freight train.
Yup, it is overpowered for mule deer but "shock and awe" is what America is all about! Heck, Lungpopper hunts everything that walks with a .338 WM!
You made a great choice! I will always own one and can't begin to explain how well it works on game. Keep us posted on your results. -
And besides, if it doesn't work out you can always trade it in on a .260 Rem! ;D
For a good comparison the 300WSM is the 4th cartridge from the right and the 300WM is the 5th from the right...