That's quite odd, BK......my factory 300 Wby Rem 700 doesn't seem to weigh any more than my other long action 700s, the same goes for my Rem 700 257 Wby. The 340 Win supergrade is a tad heavier than the 700s, oh ya and they all shoot 3/4 moa and better. These generalizations just aren't meshing with my experience at all, even my old Mark V 340, which I assume you are referring to, would shoot little 3 leaf clovers with 250 Parts when I did my part at the bench.
Again I suspect you haven't worked with Wby cartridges or rifles very much, would that be a fair generalization BK?