Hello Silver Tip,
For the larger thin skinned animals like eland, zebra, waterbok and some others, my personal minimum is a .33 caliber, with 250 grain round nose soft at 2400 fps (.318 Westley Richards / .338-06).
That is not to say I'm suggesting there is anything wrong with a .27 caliber for those animals, except that a .27 for those larger animals is wrong for me personally.
My thing with all of this is that most critters seem disinterested in standing perfectly broadside for me when I am out hunting.
And. the quartering toward me shot requires I smash through the shoulder bone to hit heart/lung well.
On the large animals, that is a concrete block wall to defeat and a larger diameter/heavy bullet, @ moderate velocity in my experience is the best way to get through.
Whatever caliber you settle on, it is right to ask your PH about it and also what bullet brand and weight he or she recommends.
Regards,
Velo Dog.