If done correctly the CO2 will knock them out without feeling anything, too much and they will feel a burning sensation as acid forms in their lungs and eyes for a split second before they are knocked out, too little and they have a chance to escape.
It's more humane than the gopher bombing my brother used to do for the city with SO2 producing bombs that would burn their eyes/lungs till they died from shock. Occasionally he had 'shovel kill' one that dug through the plugged holes.
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