.338 win mag? very good choice with 225 or 250gr, or .30-06 with 200 or 220grain. factory rounds should be available if the worst happens... Your shot opportunities are unlikely to be more than 250 yards, probably much less and heavier bullets are my recommendation. .257 AI may be a bit light and fast for the terrain and animals you will encounter. Having said that, if you want a 2nd rifle that will be suitable for buffalo if your main rifle fails there is only one sensible choice - the .375 H&H. JMHO