Step 1: Materials
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*