RE: .257 Weatherby Magnum.
I've ran a bit of H4831 in my Roy with 100's and found it to be very consistant, however I found it's velocities trailed those of H1000, Retumbo, and R25 by a good 100fps or so with the various 100's I tried.
As a side note I found that though Retumbo is slower than H1000 they both maxed at the same charge weightwith similar velocities. In my 300Rum the difference between the two powders is much more noticeable.
I would say H4831 would be a very solid powder to start and maybe end with...
Brett