This is a project i am currently working on - spring powered BB/Dart gun. Requires Spring, Metal/PVC pipe (Whichever you feel more secure with, i chose metal), Metal barrel, I chose 4.5mm .77 barrel from an old airgun, spring, some cutting of wood, and dowwel. This is alot more powerful than other bb guns powered by combustion, bike pumps ect. so be carefull.
Step 1: Materials
Airgun Spring - Check your local airgun shop, or if your australian, find one.
Wooden circles, cut with a hole saw
Metal tubing, to fit your spring.
Endcap for the tubing
4.5mm copper, or any other metal tubing for barrel, 4.5mm is .177 for metal bb's or airgun darts, although if you wanted to fire plastic bb's you will need 6mm tubing i think.
I think thats it *thinking face*