I first heard about this rifling on the outside of the barrel stuff some time before it appeared in the books mentioned above.
I personally have never seen this but a man told me that he had seen it and this bloke has an immense amount of experience with a hell of a lot of doubles.
I'll go right out on a limb here and say that in my opinion this man would have shot more big Aussie game with doubles than anyone else on the planet. (mainly buffalo and big boars and scrub bulls)
He knows what he is talking about and had nothing to gain by making the story up.
I do believe that it could occurr, but IMO it would take more than just one or two shots.