What's the best weather-proof, tough, reliable rifle chamberd in .300 WSM. Lighter weight is better.

There is no such thing as a "weatherproof" rifle... Though there are some stainless rifles with synthetic stocks that may endure harsh conditions and neglect better than others. But stainless will rust if you negelect it. In any case, both Winchester and Browning make a nice synthetic/stainless rifle. I prefer the M70 action but that's simply a matter of personal preference.

If ruger makes their stainless rifle in the short mags I would go for that. Syn. stainless might not be totaly weather proof but they sure take a beating better then blued ones.