A CO2 powered gun does not need to be manually cocked in order to fire, as this is done from the compressed air in the CO2 cartridge. A homemade CO2 powered gun that will shoot BB pellets can be made from a few supplies found around the house, along with a few items from a hobby store. Adult supervision is suggested when making and using the homemade CO2 powered gun if the person is underage.
Place a sheet of newspaper on a table to protect the surface. Use pliers to pull the point off of a disposable plastic pen. Pull the back cap off of the pen with the pliers. Use pliers to pull the ink tube out of the front of the pen. Dispose of the ink tube, back cap and pen point.
Unscrew the top from the CO2 pump. Insert the CO2 cartridge, nozzle side up, into the CO2 pump. Screw the top of the CO2 pump back on.
Apply bonding glue to the front end of the pen. Place the non-threaded end of the inner tube valve over the front end of the pen. Let the bonding glue set for one hour.
Screw the threaded end of the inner tube valve into the top of the CO2 pump.
Go outside. Insert a BB pellet into the free end of the pen. Aim at a bush or some other surface that won't ricochet the BB back at you. Press the trigger on the CO2 pump to fire the BB from your homemade CO2 powered gun at the bush or other surface.
Never aim any kind of gun that shoots a projectile at a person or pet.