Our school library had "Karamojo Safari" and I read it in high school. Have read quite a bit about him since.
Bell primarily used the 6.5 Mannlicher Schoenauer (unsure of the spelling) and .275 Rigby (7mm Mauser). He used the long parallel sided round nose fmj bullets because he thought penetration was the most important thing in killing elephants and other large game.
I have some of those old type bullets in 6.5 Swedish Mauser and their penetration is astounding. They will shoot right through a large (very large) tree trunk.