DIY Co2 Chamber.
There are many methods of dispatching prey. Cervical dislocation, Co2, the smack method (either grabbing the prey by the tail or putting it in a sack and "smacking it against a hard surface), and the thump method (using a hard object to hit the prey on the head). While many of these methods are used the only ones considered humane are Co2 and cervical dislocation.
Cervical dislocation involves putting an object such as a dowel, behind the head of the prey, and then yanking sharply on the tail to dislocate the spine. This immediately severs the spinal cord and kills the mice or rat. There are 2 problems with this method. First ... It takes practice to get it right, while learning you usually just injure the animal and upset it greatly. The second problem is that its extremely easy to "skin" a rats tail. The skin on a rats tail is quite thin and can and will come off of the tail in your hand. This is EXTREMELY distressing to the rat (this normally only happens on rats), and there is nothing worse than having a severely upset rat by the tail.
The next method, and the one I prefer, is Co2. Co2 can come in a canister, like one used for paint ball guns, from a mix of chemicals, or in the form of dry ice. I will explain the dry ice method below as it is the cheapest to set up and run.
Find a "tupperware" type container with a matching lid. I got this one from a dollar store. Drill holes in the lid, go slowly so you don't crack the plastic. Here the smaller holes are 1/4" and the larger one is 3/8". This is the inner container.
This container is approximately 8"l x 8"w x 4" deep. It is used to hold the dry ice and water mix. It is used solely to keep the prey from coming into contact with the dry ice, doing so usually results in the loss of limbs or fur.
Next comes the outer container. The easiest item to use is a 4 or 5 gallon bucket. Again you need a lid for this ... however you don't want a tight fit. The inner container will go into the bucket once it has water and dry ice in it. An example is shown below.
Next comes the dry ice ...
Find a place to buy dry ice ... I buy it at my local grocery store for $0.69 per lb. Buy approximately 1/2 lb for every 10 rats or 20 mice.
Make sure that you have the inner container, outer container, dry ice, and prey nearby.
Once you have the required amount of dry ice its time to break it up. Break any piece of dry ice over 1" x 1" x 1". I use a hammer (the more pieces of dry ice you have the more surface area there is to produce the Co2). Set this aside for a moment.
Now grab your inner container (the "tupperware" type container). Put about 2" of water in it (or about half full). The pour the dry ice into the water. This will cause the dry ice to instantly start sublimating (turning from solid to gas). Place the lid tightly on the inner container, and set it into the bucket. Loosely place the bucket lid on the bucket (do NOT make it airtight).
Start placing the prey into the bucket. I do 3 rats or 5 mice every 15 seconds. The prey will be unconscious within 10-20 seconds and expire within 2 minutes.
In this setup pictured they fall to the side of the red tub. I normally do up to 20 rats or 50 mice in the 5 gallon setup.
With this particular process do NOT use an enclosure with thin plastic sides (like the IRIS see thru tubs). The rapid chilling from the ice will crack it.
If gas flow slows down you can shake the smaller tub, but it will usually require the addition of more water. Remove the smaller enclosure and add water away from the bucket (water in the outer container will freeze and can damage your end product). It should immediately start sublimating again.
Here are some alternative containers that I use for either large amounts of rats and mice or for rabbits (this will do about 50 rats, 100 mice, or 10 - 4-5 lb rabbits).
The inner container is the same 5 gallon bucket used above. I have a 2nd lid for it that I have drilled. The outer container is a 33 gallon garbage can.
Dry Ice can cause contact burns ... you should wear gloves when working with it. I break it up in paper grocery bags, then just pour it into the inner container.
Do not make either container "air tight", the gas is constantly expanding and will blow the lid off if airtight.