The 300 WSM or 300 Win. Mag. sure as heck packs plenty of punch at 400 yards. My choice of the two in the 300 Win. Mag. I have had both. The 300 WSM rifle that I had was a Winchester Model 70 in SS with their Mauser style extractor. Try as I might, I never found a factory load and never could develop a hand load that grouped great. I have a Rem. 700 in 300 Win. Mag. that is a fine shooting rifle using a couple fo factory loads that I have tried, as well as a couple of my hand loads. It would probably be easier to find a wider variety of factory ammo for the 300 Win. Mag.

I am of the opinion that about any modern factory rifle should do OK. The two that you have mentioned would be among these. I will say that unless you are after some really heavy critters, a good hunting grade bullet out of a 30.06 Spr. or 308 Win. is plenty lethal at 400 yards.