i like making making K'NEX guns so I thought I would upload one. This uses rubber band balls or any missile you can get. But please, please, PLEASE do not use any thing that may hurt people. This is a pretty reliable gun though on one occasion I was using a K'NEX missile and the firing rod stuck to the missile. You can mod it any way you want.
Step 1: Make Everything
Make everything in the picture. Pretty easy, huh. i forgot to add one rubber band.
Step 2: Put Everything Together
Again, follow the pictures. Put one rubber band on where shown.
Step 3: Rubber Bands
Add the rubber bands as shown.
Step 4: Band Ball
Get 2 rubber bands. Double one over the other one until it's tight (picture 1). Then take the two ends and double them over again. Then double more rubber bands on. Use five rubber bands.
To double one over, stretch out the rubber band. Twist it once. Overlay it. Done!
Step 5: Shooting
Pull back the firing pin. Put missile or ball in. Pull trigger.