When we look at the .300 WSM vs. .300 Win. Mag. comparison, it looks on the surface like the .300 WSM might actually outperform the .300 Win. Mag. But that's misleading. Factory pressure for the .300 Win. Mag. is 64,000 psi while the .300 WSM's pressure is 65,000 psi. That's not a huge difference. With 180-grain bullets, the race isn't even close. The .300 Win. Mag. is the hands-down winner. With 150-grain bullets, the .300 WSM shows a 10-fps advantage (3,300 fps vs. 3,290 fps). The pressure difference more than explains this trivial velocity difference. Despite claims to the contrary, the .300 WSM does not outperform the .300 Win. Mag., and it really shouldn't since it is a little bit smaller.

Read and weep!
http://www.gunsandammomag.com/techside/myth_0903/