From http://www.african-hunter.com/stopping_power.htm by Ganyana:

"In all the sitreps I have been able to look up where a hunter has been surprised by an elephant in thick bush, the 11 cases where the hunter has been armed with a .375 he has been either injured or killed. As the rifles become more powerful, the odds improve remarkably. A .470 (or .465, 476 etc.) gives the hunter better than 50/50 odds of escaping unscathed (eight records: one killed, two injured). With buffalo it’s a similar story. A .375 solid on a frontal chest shot will seldom stop a charge dead. Both barrels from a .500 are virtually guaranteed to."