If the bolt or portions thereof ejected out the back of the rifle, then not only were pressures very high, but something else mechanically failed.
I once had a couple boxes of .25-06 factory ammo that were 300 fps shy of spec on velocity. I suppose if they make that kind of error, a different factory could also load some 300 fps to the high side.
Didn't Weatherbys used to have a long freebore, ahead of the chamber, to hold down pressures?
metal parts can fail unexpectedly for three reasons, under normal stress. Non-metallic inclusions (impurities), mechanical defects (voids or seams), or heat treating mistakes. Could be the brass case or the metal parts of the action.
Sounds like two or three things went wrong at once to cause this accident.
Edit to add, I looked at a lightweight Mark V with synthetic stock today, in .300 Winchester Mag. I still thought it was a pretty cool rifle.
