DBoston,
P.O. Ackley also claimed to have encouraged Weatherby to use longer "freebore". No doubt there were lots of influences--all (as was typical of the time period) concentrating on getting the absolute top muzzle velocity.
But the powders back then were also more limiting. IMR4350 was by far the slowest-burning rifle powder when it was introduced in 1940--and made the Weatherby rounds possible. (Actually, in a bit of trivia, it actually appeared in 1939--but wasn't distributed to the public until 1940. But in '39 you could drive to the DuPont factory and buy it. Don't think any powder company offers that option today for handloaders.)
Of course, after WWII handloaders could buy "war surplus" 4831, which was slower yet.
