This a small K'nex crossbow, I made after my brother made a much, much bigger one.
Here is a quick YouTube video I made of the crossbow.
This one uses eight 4" long rubber bands. It shoots a ping-pong ball about 30 feet far.
One piece you really need is one Big Ball Factory Tubing. Otherwise it may not
shoot as far.
Step 1: Small K'nexCrossbow
Here is how to build the crossbow, just build it exactly as instructed.
Step 2: Elevation Help Piece. Pt 1
This a simple barn shaped piece that will be used to elevate the whole crossbow. You will need to make two of these.
Step 3: Elevation Help Piece. Pt 2
On one of them you add these pieces.
Step 4: Elevation Help Piece. Pt 3
Now attach them. This is the finished part that elevates the whole crossbow.
Step 5: Upper and Lower Rail. Pt. 1
Now for the lower rail. To construct that you start off by building this. (Note: both sides are the same.)
Step 6: Upper and Lower Rail. Pt. 2
Now build these two upside down "U" pieces.
Step 7: Upper and Lower Rail. Pt. 3
Next, you need to build two of these "W" pieces.
Step 8: Upper and Lower Rail. Pt. 4
Now for the upper rail and again both sides are the same.
This is eight blue rods long and each ends in a yellow connector.
Step 9: Upper and Lower Rail. Pt. 5
Attach the "W" pieces first and then the "U" pieces to the lower rail.
Step 10: Upper and Lower Rail. Pt. 6
Attach the upper rail on the top part. Now you are done the upper and lower rail.
Step 11: The Front of the Crossbow. Pt. 1
This is the bottom part of the "limbs", you have to make two of these.
Step 12: The Front of the Crossbow. Pt. 2
This is the top part of the "limbs" and again you have to build two of these.
Step 13: The Front of the Crossbow. Pt. 3
When you attach the top and bottom "limbs" together, you need to add spacers and wheels on the ENDS of the triangles in this order: Big spacer, small spacer, wheel without tire, small spacer, big spacer.
Step 14: Attaching the "wings" to the Rails.
Now add the "limbs" to the rails and this creates the arrow shape.
Step 15: Connecting the Rubber Bands to the Crossbow
As I said in the intro, you need eight 4" long rubber bands.Join the rubber bands as you see in the picture and slide in the orange connector in the middle rubber band.
Step 16: Connecting the Rubber Bands to the Crossbow
Stretch the rubber bands as you see in the pictures.
Step 17: Connecting the Rubber Bands to the Crossbow
Make sure the orange connector is in the middle of the rails like this.
Step 18: Completing the Crossbow
Remember to attach the two green pegs that are shown in: Upper and Lower rail. Pt. 1. to the gray connectors shown in: Elevation Help piece. Pt 2.
Step 19: Launching the Crossbow
To launch, you will need a ping-pong ball. Place the ball so that it rolls down the rails, then grab the rubber band knots closest to the orange connector and pull back. 3-2-1 Fire!