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

You need,

Connectors:
2 double ended orange/brown
4 single clip gray
1 two clip gray white

Rods:
2 small blueish silver rods
? small black/green rods (for ammo)

Other:
rubber band(s)
sand paper

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