Well, he had the skill to be able to use a light cartridge.

He liked to carry plenty of ammo on his person, and carry less weight.

He didn't like really loud rifles, as they spook game more.

He liked a short bolt throw, for faster follow-up shots.

He liked a light rifle, ergo the short action preference. That shaves a lot of carry weight.

Most of these things would lead you to the .308 over the 30-06, as he really didn't have a preference for the heavy bullets.

I'm curious what he would have made of the 6.5x55. Availability in Africa would likely have killed that idea though.