I can help you make up some plans. But you'll have to answer some questions.
- What kind of projectile would you want to shoot?
- How much money are you looking to spend?
- Size constraints?
- Over under/inline/coaxial?
(Over under is where the gun makes a U shape, inline it is all "in line" and tends to be long, and coaxial the chamber is around the barrel and is sealed with a diaphram or piston valve.)
I know what he wants....