He had a 6.5 Creedmoor, sort of, and used it to kill most of his "meat" game. It was a .256 caliber which is the bore, not the groove, if I remember right firing 156 grain RN softs and solids, and he had both a short and long rifle. He found those solids to bend on Elephant skulls too often but he did shoot a few with it. He also killed Giraffe at 500 yards with it seeing with his internal "eye" the trajectory of the bullet over the screen of bush down into the vitals.
Been a few years since I read the book but think my memory serves. WDM Bell was one of those people who have the true Gift of shooting. I have met precisely one of those in my lifetime. He is now passed on. There are many good and even great shooters but few who have the Gift of shooting.