This is a K'nex rubber band gun I made a while ago. It is based on a design from a book on how to make guns out of Legos. I didn't have all the parts needed to make the design in the book, so I made my own version using K'nex instead.

This gun is made almost entirely from K'nex parts, with the three non-K'nex parts being a small block and two rubber bands. Also, none of the parts are modified in any way. (I prefer to avoid modded parts when possible.) It shoots up to 3 rubber bands using an escapement-like mechanism on the back.

Also, this is my first instructable, so any tips on how to make it better are welcome.

Step 1: Parts List

Sorting the parts is optional.

Also, some of these parts can be substituted for others, and the color of the parts doesn't make a difference. (Actually, I'm curious what the gun would look like in classic colors.)

Even though the project was made using metallic colored parts, the parts are listed using classic colors as these are easier to distinguish. Within the instructions, the parts are stated with the classic color, followed by the color I actually used in the picture.

Connectors

White (360°): 2

Yellow (180°): 18

Green (135°): 2

Red (90°): 7

Gray (45°, 2 slots): 4

Gray (1 slot): 6

Orange (180°): 3

Purple (135° + 3D slot): 10

Blue (270° + 3D slot): 1

Ball clips (for ball and socket joint): 2

Socket clips (for ball and socket joint): 3

Black hinge piece (without blue hinge piece): 1

Y-clips: 9

Rods

Green: 38

White: 27

Blue: 5

Yellow: 1

Other

Blue spacers: 4

Silver spacers: 4

White gear (optional): 1

Gray gear (optional): 1

Small black wheels/disks (optional): 2

Non-K’nex

Small block (about ¼” thick): 1

8 inch rubber band: 1

7 inch rubber band: 1

Ammo: up to 3 rubber bands

Step 2: The Trigger Mechanism and Rear Hook

This is the back of the gun. It is also the most complicated part of the gun, so after this, the rest is easier.

These assemble to form the escapement.
This is the assembled escapement lever.
This holds the lever.
The lever is attached to the holder with a blue spacer on each side.
This is the lever and holder so far after assembly.
These parts are needed next.
The yellow (light gray) connector holds the lever in place.
The white (silver) rod connects to the two Y-clips on the lever. Make sure it is behind the flat sides of the yellow (light gray) and purple (silver) connectors, as it limits the rotation of the lever to just that area.
These parts form the rotary hook on the very back of the gun.
This is their assembly.
The rotary hook connects to the back of the escapement.
This is what it looks like after assembly.
This is another view of the current assembly.
The yellow piece with the Y-clip connects to the back of the lever.
This is how it connects. It locks the rotating hook when the trigger is released.
These become the middle section of the gun.
They connect side-by-side as shown.
The rod with spacers and yellow connector are needed next. They form the side of the gun.
This is how they connect. Note that the yellow connector is above the arched side of the purple 3D connector.
This is the trigger and its associated mounting parts.
Here is the pivot for the trigger, partially assembled.
This is the fully assembled trigger.
The trigger connects to the side section of the gun.
When connected, the flat sides of the yellow (light gray) connectors face towards each other.
The trigger then rotates into the assembly as shown. This is where it will be when the gun is finished.
This is simply the other side of the trigger so far.
Finally, the trigger and latch come together.
This is them after assembly.
These two parts go on the side of the assembly. They are mostly for decoration, although the purple (silver) connector helps a little with the structure.
The purple (silver) connector attaches to the lever mechanism, while the black socket clip connects to the trigger module.
Note that part of the purple (silver) connector wedges within a slot on the yellow (light gray) connector.
These parts will be needed to connect the trigger to the lever.
They connect through the hole in the yellow (light gray) connector on the trigger. Note that this view is on the side with the circular white (black) connector. On the other side, the Y-clip should point towards the lever.
This is the other side of the trigger connection, along with the next parts to be used. I have not tested a normal sized ball and socket joint, but it should work. I used the larger one mostly because it looks cool. This can also be substituted with a black/blue hinge, but I do not recommend this. The hinge has to be horizontal (with the holes facing up and down) for use, but has a tendency turn itself vertical during use. This will not damage the parts, but it will jam the gun, and you will have to reach inside to twist the hinge back to the way it should be. The ball and socket joint eliminates this problem.
This is where the joint connects to the Y-clip...
And this is where it connects to the lever on the back. You now have a working trigger!

Step 3: The Hand Grip

It's kind of hard to hold the gun without this...

These are the first pieces of the hand grip
They connect as shown. The white (black) rod goes in the slot 45 degrees from the 3D connector and underneath the pair of green (black) rods. It does not go between the green (black) rods.
Make the two identical truss pieces shown.
These both connect to the first piece. The one on the blue side aligns to the 3D slot and the one on the white (black) side goes directly under it.
This piece will form the bottom of the grip.
It connects to the other side of the truss.
These two pieces complete the truss.
This is the completed truss structure for the grip.
This green piece with a Y-clip is mostly decorative, but it does help maintain the thickness of the grip.
The Y-clip on it fits in the gap between the two purple (silver) connectors.
Here is a side view, showing how the parts align on the bottom.
This gray piece with a Y-clip is also mostly decorative.
This is where it attaches to the handle.
The handle is now complete.
It attaches to the previous module via the two yellow (light gray) and the purple (silver) connectors on the bottom. Don't worry that there are only three connection points for the grip. That is intentional. The last rod (on the opposite side of what is shown in the picture) is for decoration.

Step 4: The Barrel

Here is the front hook for the rubber bands, along with some more decorative and structural elements.

This piece isn't part of the barrel, but it will be necessary for attaching it.
It connects to the front of the first module.
Here it is after connection. The piece below will keep it at a slight angle, but that won't affect anything structurally.
These parts form the next structural modification to help hold the barrel.
They connect as shown. The white (silver) rods go on the purple (silver) connectors while the white (black) rod with two Y-clips goes on the two yellow (light gray) connectors.
Now to finally start on the barrel! Note that the two trusses are not identical. This is important.
The white (silver) rod holds the trusses together. This picture shows the right truss connected to the sight.
They are now fully assembled.
The blue rod, gray clip, and Y-clip will be necessary for attaching the barrel to the rest of the gun.
This is how they attach. The blue rod goes through the holes in the gray connectors on the barrel, and the extra gray connector clips to the rod between the two trusses. Note that it is on the opposite side of the rod from the Y-clip.
The barrel is now ready to be joined to the rest of the gun.
The yellow (light gray) clip on the barrel goes between the Y-clips that were added to the first module earlier in this step. This will lock that angled yellow (light gray) connector in that angle.
Despite appearances, the barrel is not fully attached yet. Trying to add rubber bands would bend it all the way off.
This piece will give it the structural reinforcement it needs. The ball and socket hinge can be replaced with a black/blue hinge without affecting performance.
The yellow (light gray) connector on one end of the new part attaches the the Y-clip on the barrel, while the green (black) rod on the other end connects to the green connector on the back of the trigger mechanism.
These will finish the barrel connection. The red (dark gray) connector is semi-decorative, and may be replaced with almost any clip or connector if needed.
These parts connect to the other side of the blue rod as shown.
This will provide a small amount of compression support.
It attaches to the purple (silver) piece from the first module. The flat side of the black hinge piece faces out, and the end touches the yellow (light gray) connector.

Step 5: Rubber Bands and Finishing Touches

Here is the part that actually makes the gun semi-automatic. It is also where the non-K'nex parts come in.

This is the small block. Mine is the lid off a box of mechanical pencil leads, but anything about 1/4" thick should work. That said, stacked coins is a bad idea. They slide around and eventually fall out. Also, if you do not care whether or not the gun is semi-automatic, you can leave this part out entirely. Doing this will cause the gun to fire all the rubber bands as soon as the trigger is pulled.
Here is the 8 inch rubber band. It hooks around the back of the lever and loops under the blue rod with the spacers. The block in step 1 goes between the rubber band and the lever.
The 8 inch band then loops back around to the small block on the back of the lever, securing it to the back.
The 7 inch rubber band starts behind the back of the small block and loops around the gray 45 degree connector on the barrel.
This band then wraps underneath the barrel and connects to the blue rod.
This is a closeup of the small block. Notice that the 7 inch band (the blue one) wraps behind the block and over the ends of the white (silver) rod. This will secure the block against the impact of the hook.
These parts are entirely for decoration.
The white gear and extra green (black) rod act as a thumb catch for loading. This isn't necessary, but it makes it a bit easier.
It attaches right beside the Y-clip on the back of the latch.
This is just to make the gun look cooler. Experiment to see what you think looks right.
The same goes for the black disks on the handle, although their location does present a slight restriction. Since they are on the handle, the wrong parts may not feel right when held. Again, experiment with this to see what looks and feels right.

Step 6: Finished!

The gun is finally finished! You can now load and fire up to three rubber bands from it.

To load the gun, you may start from either the front or rear hook. The pictures show loading from the front hook, but loading from the rear is just as easy. When loading, make sure the rubber bands go between the purple (silver) connector on top and the gray 45 degree connector on the bottom. This will prevent it from catching on those parts as it fires. Also thinner rubber bands (like the red and yellow ones in the pictures) may get between the spacers on the rear hook and slow it down. If they do this while you are loading the gun, pull them out and twist them a bit to the side to keep it from slowing down.

To load multiple shots, load a rubber band, then pull back the rear hook backwards with your thumb or finger. Use the trigger or thumb catch to pull the latch out of the way as you rotate the hook. Once the hook is 180 degrees from its original position, release the latch. This will hold the hook in place. A new rubber band can now be loaded on top of the first one. This can (in theory) be repeated any number of times, but the structural limitations of making this out of K'nex reduces the number of rubber bands it can hold to three. Adding more than that causes the barrel to bend sideways.

I hope you enjoy this. It was fun to build, and I hope it is for you too.