Since I made that post I have researched the body cavity injection. To my mind it is not an acceptable practice for euthanasia, unless the animal is anaesthetised first. If the animal is rendered unconscious then is injected in the one of the vital blood carrying organs, I do not have a problem with it.
I do not believe that you can push a needle through an animals organs without causing that animal pain, as in the heart injection from the body cavity, it would have to travel though the diaphragm and the pericardia to get to the heart. Although I have no proof that the animal would feel it