An addendum re: Zim rifle requirements:
For the smaller ungulates (Impala, klipspringer, etceteras), the minimum legal bore diameter is 5.56 mm.
For just about everything else (except elephant, hippo and buff), the minimum legal bore diameter is 7mm.
For the big stuff, the minimum legal bore diameter is 9.3 mm and 5,300 joules of energy.
Except for the smaller ungulates, that pretty much rules out a 6.5x55 or a .270 as a "legal" bore diameter, even though they have a fine record as big game cartridges.
If your PH hunts concessions leased from the government, as you know, he must have a government game scout with him to ensure that all hunting laws and regulations are followed. Most game scouts are decent fellows who enjoy the hunt, help out whenever possible and look forward to their tip at the conclusion of the hunt. However, I hunted with one humorless game scout who enforced every rule and regulation to the letter. That's a good reason to use 7mm as a minimum for the rifle you are gifting to your friends.
I say again, for a myriad of reasons, you can't go wrong with a .30/06.
