Brad,
Actually, my experience is the average factory .300 WSM isn't any more accurate than the average factory .300 Winchester Magnum, probably because the imperfections in factory rifles don't allow for the best out of the .300 WSM. Even with custom rifles, the .300 WSM's edge is so slight it's not enough to make any difference on 99.5% of hunting shots, even at extended ranges.
And a year or so I did an article that involved research into what cartridges various military and national police organizations around the world use for their sniper rifles. Most use the .223 or .308 for shorter-range stuff, and cartridges from the .338 Lapua to the .50 BMG for super long-range shooting. But for general long-range shooting the overwhelming choice is the .300 Winchester Magnum. In fact only one used the .300 WSM.
This may be partly because the .300 Winchester has been in use for so long (it's now 50 years old), which means ammo and rifles are easier to come by, and somewhat standardized. But they also apparently have plenty of faith in the .300 Winchester when it comes to hitting people at 1000 yards.
In fact last fall I accompanied a semi-retired Navy Seal sniper while hunting on a ranch in Texas (semi-retired because he still gets called on for occasional special jobs). The ranch not only offers hunting but does classes in long-range shooting, and this guy does some of the teaching, and also once in a while takes out a cull animal, usually so they won't breed with other animals. But the ranch also really represses the pig population, so that's part of the deal too. His own rifle for this work is a custom .300 Winchester Magnum.
Now, the accuracy advantage of the .300 WSM can be quantified in indeed labs and in long-range target shooting--though these days most long-range target shooters use something that kicks less. But in the real world the difference is slight.
