Also, IME, CO2 comes out pretty damn cold on it's own. I believe it cools as it expands from liquid to gas, thus the freezing/frost that can sometimes form on a cylinder. I may be wrong, as I am not a physicist...
True - except CO2 is never a liquid.
It's liquid in the cylinder due to the pressure.
Google "paint ball fill station." If you're filling canisters at home (I'm not, looks WAY to much of a risky PITA) then you need to either invert your larger canister or get one that has a dip tube so that you can transfer the liquid CO2. Otherwise, you'll never get a good fill on a smaller canister.
People actually do this to save money. Would scare the day lights out of me.