The Magbow: a Magazine-fed Knex Nerf Gun

This is the first working version of an idea I had a couple years ago. It is a Knex crossbow that fires Nerf darts from a magazine. This design uses detachable 4-dart magazines and a top mounted slide action to load the darts and draw the bow.

Step 1: Parts Count

Here are the parts used in the Magbow. Of course, sorting them out is optional, and some of the parts can be substituted for others. The first number in each entry shows the number of parts required for the crossbow itself, and the second number is the number of parts for just the magazine.

Connectors:

Yellow connectors: 26 - 6
Green connectors: 16 - 5
Red connectors: 29 - 0
Gray two-slot connectors: 2 - 2
Gray one-slot connectors: 20 - 4
Orange connectors: 22 - 6
Purple 3D connectors: 0 - 2
Tan clips: 4 - 3
Blue clips: 1 - 5
Ball clips: 2 - 0
Black/blue hinges: 2 - 0
Y-clips: 17 - 3
Blue spacers: 17 - 10
Silver spacers: 11 - 2

Rods:

Green: 41 - 17
White: 4 - 6
Blue: 19 - 7
Yellow: 13 - 2
Red: 12 - 0
Gray: 2 - 0

Non-K'nex parts

7 inch rubber bands: 2 - 1
4 inch rubber band (relatively thin): 1 - 0
String: ~15-16 inches (Works best when cut to length after attaching to the crossbow)

Step 2: Assembling the Frame

This forms the firing rail of the project and holds it together. The final shape of the rail is what makes it Nerf-compatible.

This is a central frame piece. You will need two of them.
You also need two of these. They will form the sides of the frame.
You will need three yellow rods to attach the frame pieces together.
This shows the center frames connected to the yellow rods. They go in the middle of the yellow rods.
This shows one side piece connected to the two central pieces. The Y-clips on the bottom. connect to the rods.
Here is the assembled firing rail.
These parts will help hold the handle together.
They connect to the blue rod with the Y-clips on the outside.
Here is the bottom of the handle with all of them connected.
These pieces will go on the back of the frame.
Here they are connected to the back. The Y-clips are where the magazine will go later.

Step 3: The Trigger

Most crossbows work better with these... :-)

This is the trigger itself.
It attaches to the pins in front of the handle as shown.
The two ball clips go on the front of the frame.
They both attach facing up and to the back. These will hold the rubber band that resets the trigger.
This will become part of the bow later. It also limits the forward range of the trigger's motion.
This piece connects just under the nose of the rail.
The small rubber band for the trigger threads through the hole in the orange connector.
The ends of the rubber band go around the ball clips as shown.
A front view of the assembly so far. Notice that the rubber band loops around the yellow rod from underneath and attaches to the clips from the front. This helps with the tension.
The hinges will connect the trigger to the firing mechanism. They attach on opposite sides and both face away from the trigger.
These parts assemble to link the trigger to the firing mechanism.
Here they are assembled.
The blue rods on the link attach to the hinges on the trigger.
These two parts will form the firing mechanism.
Holding the crossbow right-side-up, the larger hook with the yellow connectors goes on the left side, and the smaller hook with the orange connector goes on the right side. The use of two different hooks helped reduce the number of jams during testing. You can also make both sides yellow or make both sides orange, but making both sides orange will increase the likelihood of jamming.
The other side. Notice on both sides how the Y-clips connect to the hooks.
Add a blue spacer on each side of the firing mechanism.
The yellow (gray in the pictures) connectors will help hold it together.
This is a closeup of one near the firing mechanism. These have two red connectors attached with a white rod, and they mirror each other.
All four connectors attached to the frame. They all point backwards, towards the firing mechanism.

Step 4: The Frame

Just a quick step to add stability to the structure.

This will form the forward slide for the reload mechanism.
It attaches to the front end using the yellow (gray) connectors from the last step.
The rest of the frame is just two blue rods and two red rods.
The blue rods attach between the yellow (gray) connectors, and the red rods attach to the ends.

Step 5: The Bow

The bow is hinged, and this forms part of that hinge.
The green connectors go on the blue rod to form the other part of the hinge. The other blue rod with the tan clip will hold them together.
The blue rod attaches to the second slot from the left on the green connectors. This should hold them together and limit their range of motion.
A third blue rod attaches to the other side of the yellow connectors. It contains a silver spacer and blue spacer between the yellow connectors.
Two blue spacers go on each side of the blue rod, and they are held in place by a tan clip and gray connector. (These can be substituted for other connectors, preferably with one slot.)
Two yellow rods and two red connectors form one arm of the bow.
The arm attaches to the center slot of the yellow connectors. The corners of the red connectors face away from the hinges.
This is the arm attached to one side of the frame of the crossbow. Important notes:
1. The frame connection and the blue rod are separated by a slot on the green connector.
2. The yellow rod with the orange connectors is not used. The other two yellow rods are used to connect to the bow.
3. The rod with spacers points forward, away from the rest of the crossbow. This is important for attaching the rubber bands.
A red connector is attached to the hinge rod as shown to help the motion limiting. Early models without this would occasionally shoot the gray pieces backwards.
Steps 1-9 are mirrored on the other side.
This is what the whole thing should look like at this point.

Step 6: The Slide

This is the beginning of the reload mechanism.

These are the parts for the first and center part of the slide mechanism.
The center of the slide is assembled. The yellow rods form a lid to keep darts from being ejected out the top during loading.
This is the front end of the center piece. The sighting bead in the center (the Y-clip) is purely decorative and is optional.
The assembled center slide, with four extra red rods. These are what will slide when it is completed.
The four red rods simply connect to the ends of the center slide.
These will guide the bowstring. They are mirrors of each other when assembled.
The completed slide. The gray connectors on the bow guides must be towards the forward end on both sides.
The rods on the front of the slide slide through the gray connectors on the front of the crossbow.
This will connect the slide to the back end of the frame.
The yellow connectors attach to the red rods on the back of the frame, and the red rods on the slide go through the holes on the gray connectors.
This will form the back of the slide. The spacers are important.
Here the back end is assembled.
It attaches to the back of the slide to form a pull handle. The red connector faces down toward the frame.

Step 7: Adding the Bowstring and Rubber Bands

The source of power. This is the last step in making the crossbow itself.

The bowstring can be tied to the blue rod using any knot that doesn't come untied when yanked. I just used a bowline with a loop large enough to slip easily onto the rod.
The green connectors attach to one arm of the bow.
The bowstring threads over the gray rod on the slide, under the two red (metallic green) rods, and over the other gray rod. It also goes behind the two gray connectors, as shown in the picture.
This is a closeup view of the threading pattern. The string goes over the gray rods and firing rail, but under the red (metallic green) rods. It goes behind the gray connectors.
Another blue rod with green connectors holds the other end of the bowstring to the other end of the bow.
Rubber bands are added to the front of the bow. The pictures and part count shows two rubber bands, but if you are willing to remove and reattach a blue rod on the front of the bow, you can add one or two more to increase the power. The rubber bands loop on themselves once, as shown in the picture.
This is the front end of the bow with both rubber bands added. The crossbow is now capable of firing darts.

Step 8: The Magazines

The bow may be capable of firing darts at this point, but the magazines were my original reason for building it. These store darts for loading into the bow.

The dart pusher is technically part of the crossbow instead of part of the magazine, but the crossbow itself works without it, so it is here instead. It is simply four orange connectors and a blue clip attached using green rods. The clip on the end must be blue (without a pin, unlike the tan clip). Regardless of how many magazines you make, you will only need one dart pusher.
A magazine starts with two identical frames like the one in this picture.
One frame will get this finger to hold darts down. The finger consists of two white rods, a two-slot gray connector, and a one-slot gray connector as shown.
The finger attaches to the top of one frame at the green connector as shown. The holes in the green connector and gray two-slot connector should align.
This is the back wall of the magazine. The tan clip may be replaced with almost any other connector or with a pair of blue spacers, but it must face down relative to the orange connectors (unless you use spacers, in which case orientation is irrelevant).
The back wall will be attached to the other frame piece, along with a blue rod with a silver spacer and a blue spacer.
Both of these attach to the other frame piece on the bottom (the yellow connectors). The orange connectors should press against the frame, and the hole in the gray connector should line up with the hole in the green connector.
The two frame pieces are now connected using the two blue rods on the bottom.
Another view of the magazine at this point. The back wall and top finger should both be between the two frames.
A back view showing the alignment of the parts on top.
A silver and a blue spacer on the back side help press the frames together.
This piece will hold the top of the frame together and guide darts onto the firing rail. The blue clips are important. The connectors on the ends are temporary spacers and will be removed later.
The Y-clips on the guide connect to the front of the frame so that the blue clips face down, into the magazine.
This rod will form a pulley for the rubber band.
The rod slides through the holes in the green connectors, and the gray connector lines up with the white rod as shown.
Adding a clip to the back end secures it. This should be a tan clip or, preferably, a blue clip. Longer connectors will make reloading more difficult. The pin on a tan clip may interfere with reloading, but would not interfere if the clip is rotated correctly.
These form the moving bottom of the magazine.
The assembled bottom. The tan clips face inward towards the orange connectors, but the pins face away from the center. The clips can be substituted for blue clips or other connectors.
The bottom slides into the frame from the front, as shown.
When the bottom is inserted, the green connector should stick out of the slot in the back wall.
This is part of the rear slide. it is a blue or tan clip, a gray two-slot connector, and a gray one-slot connector on a white rod. The order and orientation shown in the picture is important.
The other part of the rear slide.
The Y-clip attaches the the gray connector as shown, and the white rod sticks out towards the blue clip.
The slide attaches to the back of the floor as shown, so that the blue clip rides on the back wall, and the gray connector rides on the side of the frame.
Another view of the rear slide.
This is the front of the magazine with the temporary spacers removed. It should be rigid enough by this point to no longer need them.
This is what the magazine should look like at this point. The only thing missing is the rubber band.
The rubber band starts at the front end of the floor of the magazine.
From there, it threads through the hole between the green connector and the gray connector.
This is a closeup of the hole. The rubber band should bend downwards so the spacers act like a roller.
Finally, the rubber band connects to the bottom rod in the front. If the crossbow doesn't load properly, and the tension here seems to be the problem, different attach points can make the band tighter or looser. The band can also connect to the back of the floor instead of the front, although this was a bit less reliable in testing.
The magazine is now complete. You can make any number of these.

Step 9: Loading and Firing the Crossbow

Now for the fun part.

To load and attach a magazine:

Up to four streamline darts can be loaded in a magazine.
Pulling down the slide allows you to load three darts through the front of the magazine.
The fourth dart is loaded from the top.
Here is a magazine with all four darts in it.
Here is the same magazine from the top.
The magazine attaches to the rear of the crossbow, in the hollow space.
To get it into the space, you have to tilt the magazine sideways and feed the blue rods through one side at a time. They are too long to fit in going straight.
The blue rod on the front of the magazine attaches to the two Y-clips on the back of the firing rail. The easiest way to do this is to rotate the back end of the magazine upwards, press the front against the Y-clips, and rotate the back downwards while pressing forwards. This should twist the rod into place. It isn't the easiest loading magazine, but it gets better as you get a feel for it.
Pull back the slide and insert the blue tip of the dart pusher into the slot on the back of the magazine. The other end attaches to the red connector on the slide. Removing the magazine is simply doing these last three steps in reverse. Twisting the magazine on the second step makes it come out easily.
This is a view of where the dart pusher attaches to the crossbow.

To load darts from the magazine:

On a newly attached magazine, simply push the slide forward. The bow has already been drawn, and pushing the slide forward will cause the dart pusher to eject the top dart onto the firing rail from under the bowstring.
When the slide is pushed all the way forward, the dart will fall into place in front of the string.
If the slide is forward, pull it back to draw the bow. This will also retract the dart pusher.
If the dart pusher does not catch a dart as it is moving forward, you may need to lift the magazine floor manually and/or move the slide forward and backward a few times until it catches. (This is an area that can still use improvement.)
A picture of the back side during a jam. Once it has been pushed up, it should stay until the pusher rod catches the dart.

To load the magazine without detaching it:

Pulling the floor of the magazine up with the crossbow upside-down allows you to load darts through the front of the magazine without removing it.

To Load single shots:

To load single shots, pull back the slide, drop the dart in front of the string, and push the slide forward again.
This is a way to shoot non-streamline darts such as plunger tips.

Please let me know any suggestions or improvements. Enjoy!