Didn't even know co2 shotguns existed.....must either be horribly inefficient with the co2 or very slow muzzle velocity and sort range? Kind of neat though.
Aluminum can be tough to work on, I think your best bet (without me being a Smith, though I suspect not many smiths have tried to bend aluminum barrels) would be too use medium heat around the general area where it bent, but not to much heat. My understanding is you'd want it just starting to glow a little, and put very gentle force on it.
To make sure you don't collapse it, you'd be wise to plug the ends if the barrel and fill it with sand. That way, when you bend, you likely won't crease it etc. Try using something with a large radius to bend against, something like a concrete curb or similar. Something that you can bend against and not risk creasing or denting it.
Good luck. Oh also let the barrel and sand cool naturally and slowly, I'd put it in an oven at about 250 F after you've done the bending, let the whole thing heat to 250 ish evenly, and with the door closed turn it off. Let it all cool before opening the door. Less likely to get isolated spots where the heat gradient had weakened it.
