Only have experience with one Weatherby but the results were very good. The load was with H870, CCI mag primers, and 180gr Nosler BT. With a near max load, the vel was over 3400fps with 1/2 MOA accuracy. Brass was sized using FL RCBS dies and seated using a die from the same set. Bullet length was kept so that rounds would feed from the magazine. Brass life has been good.
Another load with Speer Grand slams was horrible. 2 to 4 MOA groups. Bullet probably not designed for this velocity. H1000 was also tried but results were not good.
This rifle also shot the Factory loads with Barnes X 180 very well. Maybe just a lucky gun. No work has been done on the rifle except to mount a scope. Muzzle break left on.
I shoot a 300Weatherby in a custom rifle
. There is also freebore but accuracy is excellent. Freebore may not be that big of an evil if loads are prepped properly. I neck size and there is a very close fit in my case and seating die. Ammo is concentric.