Thanks very much for your comments Sunny Side Up! It's the "almost" in "almost before they feel the pain" that disturbes me. Maybe I should invest in a shotgun, as you can't get much more instant than that. I've spent another 5 hours researching the euthanasia of chickens today, and find that even the most civilised countries (e.g. US, UK, NZ) disagree on the best way to do it, and are still looking into it!
Carbon dioxide (CO2) gassing is definitely not acceptable to me, but carbon monoxide (CO) seems a good possibility, as CO is tasteless and odourless, and induces coma before death without any noticeable trauma or asphyxiation panic. There are of course anaesthetic gasses that will do this too, but they can contaminate the flesh, whereas, carbon monoxide does not.
. . .to be continued I expect !
Please don't do either one. With crushing the head, I forsee mistakes, resulting in a gory mess. In fact, either way it's a gory mess. With the gas, well, let me just say, if it takes that much before you'll kill one, don't.