Full disclosure.
I have a .300 Weatherby rifle. It reliably shoots .75 MOA 3-shot groups with Weatherby 180 grain spire-point factory ammunition, and my level of skill is such that I am able to replicate those groups.
I have noticed that it takes superb discipline to shoot an un-braked .300 Weatherby Magnum without flinching.
Presumably your rifle has a brake--OR you are superbly disciplined.
If not--look no further. The problem may well be human error.
As the Weatherby rifle is intended for hunting large game, presumably you have a hunting scope on it, AND presumably that hunting scope's parallax is internally fixed at 100 yards, AND presumably that scope does not allow for parallax correction--so presumably you are doing your load testing at 100 yards. And presumably you understand parallax error.
Were you seeing sub-MOA groups at 100 with Weatherby factory ammo? I'm talking BEFORE you decided to re-bed the rifle.
Presumably you have a torque wrench, and you are familiar with Weatherby's torque sequence and their torque specs?
Did you experiment with different torque on the action bolts? Sometimes one can find a sweet spot.
You might want to haunt the Weatherby boards, and make inquiries about what pet loads people are seeing success with for your particular caliber Weatherby Magnum rifle.
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Lastly--and you probably don't want to hear this--but the .300 Weatherby (for instance) isn't particularly efficient as a "tactical" rifle shooting one-ragged-hole groups, and if that is what you are looking for, you might want to reconsider. It has been designed to be a light, portable rifle an individual intent on hunting large game can take into the Idaho back country that can effectively push a hot slug through the shoulder-blade of a bull elk and fatally damage its heart. And for that purpose, a 3 inch circle of probability at 200 yards is perfectly adequate.
All the best.