The Weatherby has a generous freebore , which does reduce velocity . At least in my testing
I met a Texas gunsmith that specialized in building 25/06, 12 twist, 26" Shilen barrels. I thought he had lost his mind. Met 6 guys in camp that had his rifles.
They all shot the following loads, and I checked the velocity with an Ohler 35P:
Win case
Fed 215
100g Sierra Flat base(only!)
3600 fps, less than 10 fps ES
63g of R#25
shot bug holes
neck sized the brass afterwards
Win case
fed 215
117g Sierra flat base(only)
58g of R#25
3300 fps
shot .400
max load for this rifle
I tried to duplicate the loads in a Rem 700 and the loads were way too hot. So, the 12 twist, shilen barrel, 26" length did something very special, and the leade on the Shilen barrel was short.
The above velocities are the exact velocities of a factory mark 5- 257 Weatherby
If you try the above loads in your 25/06, back off 7 grains and work up!!!
No other 100g or 117g bullet would stabilize in this 12 twist barrel. I killed a lot of coyotes, deer, and hogs with this rifle.