This instructable shows how to build a working K'Nex crossbow. Please be smart when using this, it can be dangerous. I suggest not using sharp projectiles and not aiming at anyone. I have a few dents in my wall from early testing.

Step 1: Key

Build: build from scratch, unless otherwise noted.
Attach: adding pieces to an existing thing
Details: nothing new, just a different view

Here are all the parts and names used in this instructables.

Step 2: Parts List

Rods

15 Red
50 Yellow
82 Blue
10 White
95 Green

Connectors

2 White
73 Yellow
5 Gold
11 Green
17 Red
60 Orange
43 Dark gray
35 Beige

Misc

7 Black rod/connectors
23 Silver spacers
36 Blue spacers
2 Hinges
2 Chain links
13.5" Orange track
40" string (any string that isn't too springy, I use fishing line)

Step 3: Starting Springs

16 Yellow connectors
12 Red rods

TIP: It is extremely hard to snap all of these together at once. Start by using the minimal amount of yellow connectors to make this structure then add yellow connectors one at a time.

Step 4: Spring (right)

1 White connector
2 Yellow connectors
4 Dark gray connectors
1 Green rod

Step 5: Spring (left)

1 White connector
2 Yellow connectors
4 Dark gray connectors
1 Green rod

Step 6: Spring (center, Part 1)

1 Yellow rod

TIP: It is hard to snap a rod into three connectors at once, try spreading the connectors out along the red rod so you only have to connect one at a time.

Step 7: Spring (center, Cut Gold)

1 Gold connector

Step 8: Spring (center, Part 2)

Gold connector from last step
2 Red connectors
2 Beige clips

Step 9: Main Body

47 Yellow connectors
3 Gold connectors
9 Green connectors
4 Red connectors
36 Orange connectors
36 Yellow rods
63 Blue rods
72 Green rods

Step 10: Combining Layers and Spring

Temporarily remove the beige clips attached to the yellow beam on the center of the spring. This will allow you to shift the connectors snapped onto it around, making it much easier to connect the layers.

NOTE: Make sure the gold track guide layer is the center layer, connected to the gold track guide connector which you cut earlier.

Step 11: Trigger Housing Start

2 Yellow connectors
7 Red connectors
9 Orange connectors
11 Dark gray connectors
4 Beige clips
2 Yellow rods
6 Blue rods
18 Green rods
2 Black rod/connectors

Step 12: Adding Slidebars

3 Dark gray connectors
3 Red rods
3 Black rod/connectors
15 Silver spacers
6 Blue spacers

Step 13: Finger Trigger & Mechanism

7 Orange connectors
12 Dark gray connectors
5 Beige clips
4 Blue rods
10 White rods
2 Hinges

Step 14: Partially Assemble Trigger Mechanism

3 Blue spacers
Rubber band(s)

TIP: Experiment with rubber bands to find what works well for you. I personally prefer two #32 rubber bands (3-1/4" long when flattened, 1/8" wide)

Note the black connector/rods connecting to the white rods in the second picture.