I agree, CO2 is perhaps the most humane route to dispatch feeder rodents, and the paint ball gun canisters work great for this purpose.
Keep in mind, you can attach a small regulator valve and length of hose to the canister, which can be inserted into a small hole inserted into the bottom of the container being used to dispatch the feeder rodents.
Since CO2 is heavier than O2, you can insert small holes into the lid of the box, which will allow the CO2 to purge the O2 out of the container, without blowing the top off, as you fill the box with CO2.
I have also used plastic shopping bags and the CO2 canisters used for air guns, when dispatching a few small rodents...
Best regards,
Jeff
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Jeff Snodgres
University of Arkansas
snodgresjeffreys@uams.edu
501.603.1947