What's the point of this rifle for you (what's it going to be used for)?
Around here (MT) factory ammo is abundant.
The 300 WSM can easily be loaded to factory velocities with a number of powders.
WW 300 WSM brass can be had for around $37 (or less) per 50 pieces... not exactly expensive IMO.
I'm from the other side of the country and always hawk the factory ammo shelves just to look. I'd say the 300 WSM has plenty of options and brass isn't really anymore expensive than 300 Win. Also I never had much trouble matching factory numbers in my 300 WSM. If you like it, go for it.
