Check out this video tutorial on how to make an airgun out of a water gun. This is a very cheap way to make a powerful homemade airsoft, BB airgun.
You will need a water gun or super soaker, a McDonalds plastic straw, airsoft BB's, a hot glue gun, electrical tape and a pair of scissors.
When you pump this gun up 100 times and fire it, depending on how quickly you pulled the trigger, you would only need to repump the gun 15 times to have it fully charged and ready to fire, just so you know. Just don't shoot cats with this homemade weapon.