Smallest K'nex gun on instructables that I know of. It uses only 9 pieces. It can shoot up to 15 feet but it can be less or more depending on the rubber bands you use.
UPDATE!!! put a new rubber band on it and reinforced the gun with duct tape(also makes it look cool)and used a bb as ammo... it hit my bathroom wall 30+ feet away with a good amount of force
Step 1: The Parts You Need
2 double ended orange/brown
4 single clip gray
1 two clip gray white
2 small blueish silver rods
? small black/green rods (for ammo)
Step 2: Build the Gun
Use the picture below to build the main part of the gun
Step 3: Make the Fireing Pin
Use the picture.
Also look at the other pic. I recommend that you sand the end of the pin that goes into the barrel so it doen't catch something in the barrel and make it misfire
Step 4: Prepare to Fire