Co2 is cheap dirty and ineffective for humane inert gas asphyxiation. If you don't use argon or nitrogen it is cruel. There is no other way to look at it. Science has no opinion. This is going to be one of those terrible topics I must choose to grit my teeth over. The world is flat. Evolution never happened.

Controlled atmosphere killing (CAK) or controlled atmosphere stunning (CAS) is a method for slaughtering animals such as chickens by placing the animals in a container in which the atmosphere lacks oxygen and consists of an asphyxiant gas (one or more of argon, nitrogen or carbon dioxide), causing the animals to lose consciousness. Argon and nitrogen are important components of a gassing process which seem to cause no pain, and for this reason many consider some types of controlled atmosphere killing more humane than other methods of killing.[8][9] If carbon dioxide is used, controlled atmosphere killing is not the same as inert gas asphyxia, because carbon dioxide at high concentrations (above 5%) is not biologically inert, but rather is toxic and also produces initial distress in a number of animals.

On topic. I hold the animal by its tail and swing It hard and fast with a quick jerking motion at the wall. Try to focus all the inertia on the neck for its breaking point. Time from out of box to unconscious and being consumed is less than 5 seconds. I can link up 100 different sources about how using Co2 is terrible. AVMA recommends it because it is cheap. They don't have to source anything but c02. Which is cheap. Effective sure but so is a whole list of things other than choking them to death after the seize up from a panic attack. I already got in trouble for discussing this to the Super moderator. Facts are facts. Nazis used pure co2. Ya know, because it's humane and all.