O'connor surely loved his .270 but I think the data says better things abound. A 6.5x55 handles heavier bullets, as does a 7x57. He was thinking American but the Europeans had the better alternatives. I think your dad would agree with me.
rookhawk,
I agree with you on this.
Furthermore, I seem to recall O'Conner writing, toward the end of his Scribe career that, he actually just slightly preferred the .30-06 but, made a good living writing about the .270 (in my words: "So, he rode that wave, all the way to the beach").
Trivia: When he and his wife, Eleanor O'Conner went to hunt in Africa , she brought a .30-06.
Anyway, it remains a mystery that, when Winchester introduced their .270 cartridge, Remington did not answer it right away, by chambering their standard bolt action of the day (Model 30) in caliber 7x64 Brenneke.
For unknown reasons, it was not until decades later when the .270 had reached almost cult status that, Remington sluggishly introduced their ".280 Remington" cartridge (ballistic twin to the excellent 7x64 cartridge, which likewise pre-dated the .270 cartridge by a decent margin).
The Brenneke also enjoyed a near cult status, in Europe and the German Colonies.
Sadly, the day late and dollar short .280 was received by most N. American Hunters with a collective yawn, perhaps due to thousands of hunters already owning a .270 Winchester.
Imo, the 7x64 is well suited for the same category of animals at longer ranges that, the American .270 is, when it's loaded with similar weight bullets.
However, the gloves come off when pressing a 7x64 to hunt larger animals, such as moose or zebra in thick cover, because 7x64 ammunition can be bought with factory loaded 175 grain bullets (goes for Remington's .280 as well).
With you specifically, I know I'm preaching to the choir but, maybe there are others just getting started in searching for a decent African rifle battery, and hopefully my rant here might be of some interest to one or two new to International Hunters here.
So as to avoid several tons of hate mail from .270 owners, I do like the .270 and having owned more than one or two rifles in this caliber, I've shot deer, caribou and vermin with it.
It is excellent for long shots on small to medium sized, thin skinned game (deer / impala / caribou / nyala / pronghorn / springbok, etc.)
However, at the moment I do not own one, because much like yourself, I have both a 6.5x55, as well as a 7x57 plus, so as to not completely rip off this thread, I finally got it through my thick skull that, anything I could consistently hit with my .270 / 130 grain spitzers, I could also hit consistently with my .30-06 / 150 gr spitzers.
So, my bases are covered, so to speak.
As the above cartridges compare to the .308 Norma magnum and similar ballistic cartridges, the .30 is better suited to using bullets of about 180 grains and upwards, when hunting larger / tougher animals.
There is no chance I personally would select a 6.5 or .270 to haul half way around the world to hunt the largest African antelopes or zebra but, in the absence of a larger than .30 caliber rifle, (renting a rifle from the PH), I might consider hunting them with a .30-06 or .300 Magnum / 200 gr or heavier premium bullet (A-Frame being my personal favorite but as PHOENIX PHIL said, there are several excellent brands now).
I totally agree with PHIL and others that, newer tougher designs of bullets make the smaller cartridges more effective on largish animals nowadays.
And so, such premium bullets likewise make the .300 Magnums suited to what the .338 Winchester used to be sort of a minimum preferred cartridge for.
But, as much as I have to scrimp and save for years at a time to enjoy my few and far between hunting trips, to rip off Robert Ruark, I prefer to "use enough gun."
Blah, blah, out,
Velo Dogmatic.
It is my absolute favorite cartridge!!! 
