In several articles about Carlos Hathcock, it states that he used the 300 Magnum in competition at Cherry Point, but during his tours in Viet Nam, he used the 30-06 and .308
I couldn' t find anything about him using the 300 mag during sniping missions. Maybe someone with more knowledge about Hathcock can help here.
There is a great article on
www.snipershide.com about Carlos Hathcock.