First of all, you will need all of the things shown in the picture. Get them together.
This gun is a moderately powerful K'Nex gun, which I made to be as small and portable as possible. Enjoy!
Step 1: Build Barrel
2. Add a red connector with a green little rod in the middle slot. Put a yellow rod on the other slot, as shown in the 2nd picture.
3. Add two more grey connectors, as shown in the 3rd picture.
Now move on to building the firing rod...