The Barnes Original bullet isn't all copper so is no better or worse than a bunch of other premium bullets out there.
In my opinion, trying to take a Cape Buffalo or other dangerous game with a bow or a single shot firearm is a stunt. Typically, the intrepid hunter will shoot and his PH will "collaborate" and take a quick second shot with his double rifle while the client fumbles to reload. The PH isn't stupid and doesn't want to be killed.
Now if you are doing everything yourself and/or your PH is also carrying a bow or single shot firearm, then I say you are very brave, very stupid or very desperate.