I shot my first ever African antelope- a duiker - with an open sighted .303. Missed my second ever - a kudu - with a hopeless .30-06 that had a broken stock. I killed my first ever trophy - an impala - with an 8x68S because I wanted to try a scoped rifle! These days I'd recommend the .303/7x57/.30-06 class up to and including kudu and I'd go for a 9.3x62/x74 for sable, and bigger. One rifle for everything (plains game)? a take-down Mauser 9.3x62 or a double 9.3x74/.375H%H flanged. Of course, you should never be without a .303 - preferably a double