A paintball gun gets its power from an attached, compressed carbon dioxide tank that propels the paintballs accurately through the air. These tanks can be dangerous because of the amount of pressure inside them. If you're done playing paintball, or if you're just getting ready to buy a new tank, take precautions when disposing of your old tank, especially if it still has compressed CO2 in it. Releasing the pressure too quickly --- by puncturing or crushing the tank --- can turn it into a projectile that can cause injury or death.
The first step to disposing any carbon dioxide tank is to empty it. Because the contents of a CO2 tank are under so much pressure, they can cause serious injury or death if the pressure is released too quickly --- like when the tank is punctured or crushed. There are two ways to empty the tank. First, attach it to your gun without any paintballs and fire the gun repeatedly until there is no more CO2 left in the tank. If for some reason your gun isn't available, you can also manually discharge the tank by compressing the trigger at the top of the tank. Use caution when doing this because exposure to a large amount of compressed gas can cause frostbite or other injuries. Use a pair of gloves and a screwdriver or other long-handled tool to hold the trigger down while in a well-ventilated area.
Once the tank is empty, you can recycle it with the rest of your recyclable materials.
If you don't have access to a recycling facility, you can throw the empty tank away. Once it is empty, it no longer poses a threat.
If you decide you're not quite ready to get rid of your CO2 tank, you can take it to an authorized store to have it refilled.
You can also try to resell your tank once it is empty. Your local paintball supply store might be able to help you find a buyer or might even buy it back from you directly.