easy and works well for how long it takes

(check new step (# 6) )

Step 1: Materials

you will need:

2 liter bottle
bike pump (with needle)
cork
two 7/8" PVC valves
one 7/8" PVC "T"
one piece of 7/8" PVC 1/2" long
two pieces of 1/2" PVC 1 1/2" long
two pieces of 1/2" PVC 2 1/2" long
one piece of 1/2" PVC at least 2 feet

Step 2: Construction

using pvc cement, put the pieces together as shown in step 1 picture.

IMPORTANT:
make sure to glue all pieces EXEPT the barrel! do NOT glue the barrel to the valve as you will be taking it off to reload

Step 3: Pump

drill a 5 mm hole down 3/4 into the cork

after that, stick the pump needle into the other end all the way

Step 4: Load Projectile

take the barrel out of the valve and put your bullet in the end
(i used a marble wrapped in masking tape)

make sure its a tight fit or it might not work very well

Step 5: Launch

to launch, make sure that the valve on the handle is open and the valve leading to the barrel is closed.

then stick the cork into the bottom of the handle and pump it up.
(i usually pump 5-7 times)

when you're done pumping close the valve in the handle and unplug the cork.

after that just open the barrel valve and your projectile goes flying