It was actually P.H. Capstick that wrote about shooting elephants in the heart with a .22, not Taylor, and Capstick got the story second-hand, and over drinks. It's in his book "Safari", "A Solid Suggestion" chapter. First time was a fluke of sorts, since they weren't actually trying to kill the elephant with a .22, just sting it so it would move into a better position, the second time was the subject of a bet.
I have to say I don't have a lot of time for either Capstick or Taylor, though their books are still on my shelves. Take it with a grain of salt.