I think the Weatherby Magnum calibers have a specified freebore in the chamber dimensions. That doesn't necessarily mean that Weatherby uses freebore in rifles built for Winchester Magnum ammo.
You could experiment with seating the bullets out a little more, though you may have to use it as a single shot if the overall length doesn't fit in the magazine.
I was going to say this same thing. The long freebore is for the higher pressures from the .300 Weatherby Mag vs. the .300 Win Mag. They would be making a giant mistake if they put that freebore on the Win Mag.
Just for the "heck" of it try a box of factory ammo. And 150 gr. should shoot fine in a .300 Win Mag.