Have you contacted Weatherby?
Yes. They asked if I'd confirmed the problem using their premium ammunition ($$$). I had not.
Following that, I did find some Weatherby .257 ammo at Cabelas and bought 2 boxes. Remember, my .257 does shoot sub-moa using my hand loads. But with the Weatherby ammo, it only shot 3.3 moa. OK, I shot 6 shots and 4 of them were sub-moa but two were way out and it was not the last two (#1 and #6 were outside the group). So much for Weatherby "premium" ammo... I have 34 rounds left to give to someone :-)
I guess if I want to pursue this with Weatherby, I'll have to spring for some Weatherby ammo in 300 Win Mag (I have not seen that lately...) and even if I do that and confirm there's a problem, I'm not optimistic at getting a resolution.