"why did Mr. Weatherby go with the double radius neck? This had to cost more to chamber for because grinding the chamber cutters and reamers"
Ol' Roy was a smart man with an inquisitive mind. He understood the venturi principle and believed that reversing the gas flow might better focus chamber pressure on the base of the bullet. A venturi works best with smooth transitions in diameter so he orfered his reamers with radus transitions.
Yes, it's more expensive but the "zing" for publicity more than compensated for the extra cost, which was of no consiquence to those who purchased Weatherby rifles anyway.
Actually, the "secret" to Roy's high velocities was large powder chambers, lots of freebore and very high pressures, not the "double venturi."