Heh yup a lot of us noticed it was a bit of a stretch. It was implied that the attack was so strong that it would pierce his own body just as easily as anyone else's body. I talked about it a bit above, but without any external forces, like in this case, Naruto wouldn't be able to force the Raikage's arm into his own body like that, but with additional leverage, he could. For instance, he could have held his back with another clone or pressed him up against an earth wall or something to isolate the movement of his arm, but without that extra force to hold his body in place to properly thrust his arm into his body, it doesn't make sense physically to do what he did.I am still confused on how naruto got the 3raikage's arm to back fire on him?... it would be impossible unless he grabbed his arm and push it through.
The logical outcome of Naruto redirecting his arm with a rasengan would simply be Raikage getting slightly redirected and his arm moving across his body somewhat, but not impaling himself. Again, it was a stretch on Kishi's part, but an awesome ending to a fight.