Without a CO2 chamber, you can easily take care of it another way.
Just pick up a little chunk of dry ice. Get a 5 gallon bucket. Put the dry ice in a little container of water, with a lid loosely covering it, not closed on it. Set the lid on the bucket for a few minutes. Now just put the rat in there. The CO2 boiled off of the dry ice displaces all the oxygen and you've made a CO2 chamber. It's a very quick process once the rat is in there.
Personally, I just stun the rats (whack-em). Someone referred to it as the "sack and whack" method. But my friend will only do the dry ice thing. Whatever suits you. Thanks for the good chuckle story :gj: