Someone used to sell a CO2 kit for cooling rifle barrels. Just a medium size CO2 bottle with a regulator, valve, and hose.
Personally, I always figured a pint of water was easier and cheaper, if not less messy.
So far, I have always administered the water to the exterior of the barrel.
My tap water is fairly hard and also leaves silicate deposits which are nearly impossible to remove from glass shower doors and aquarium walls. I really would not want those deposits on the inside of a decent rifle barrel.
