Most couldn't make a 300 yard shot much less a 600.
More than a grain of truth to this.
I know hunters who have closets full of BG rifles,and have brought some of these guys to our range here, plunked them down at 300 yards.Some admit they have never fired a rifle at all at 300 yards. I know some of this is lack of access to a range,but a lot of it is just laziness, or lack of incentive, or their hunting has never included reaching that far.
600 yards might as well be on another planet.
Not all have been from back east; one week in Wyoming I went out with a guy from there and we got on a 16" pronghorn;lazed it at about 330 yards....he wouldn't shoot,and I wondered why but didn't push the issue.
Later he killed a good buck mule deer at about 275 and said that,despite living and hunting the west his whole life,that was the furthest he had ever killed anything.
I can respect anyone who knows their limitations; but under good conditions,with a good rifle and load, a solid rest and some practice,300+- yards should be a lead pipe cinch on a BG animal.
Stretching that to 600 yards,in the field,takes another level of knowledge and experience....out of reach for most without a good deal of knowledge,dedication and practice.
In theory and in the hands of an experienced rifleman who knows how to hit at that distance,I would expect a 300 WSM to capably kill anything from moose on down at distances to 600 yards.