Centrifugal Airsoft Pen Machine Gun




About: I'm a product designer, who believes in working backwards. Instead of taking technology and seeing what problems I can solve with it, I take problems and see what technology can do for the problem. If curre...

Made almost exclusively from office supplies this centrifugal machine gun is the perfect device for booby trapping beloved objects that coworkers have a tendency to take.

So it's more of an air soft grenade in this form, however with a little more time you could without much trouble get it to shoot in the same general spot each time. I still think it's great fun to play with though, even with it's shoot everything in sight mode. In the future I would like to add a much larger hopper, but for now this suits my purposes.  It's hard to see in the video, but the round had a range of about 20- 25 feet.  They didn't hurt at any range... which is good.

Disclaimer: Don't hurt yourself, or others.  

Step 1: Materials & Tools

  • 4x - large paper clips
  • 2x - disposable ballpoint pens
  • 1x - 9 volt battery
  • 1x - small dc motor (the faster the better)
  • 1x - paper
  • 1x - magnet
  • corrugated cardboard
  • aluminum foil
  • tape
  • double sided tape
  • air soft ammo or other small round object that will freely fit inside of the disposable pens.
  • exacto knife
  • needle nose pliers
  • scissors
  • awesomeness

Step 2: Build Your Barrel...

Building the barrel...

Take apart one ball point pen and harvest the body.  It should be big enough to allow air soft rounds to roll through freely. Both ends should be open.  On some brands you may need to cut off the butt of the pen to open both ends up.

Using an exacto knife or if like me you have access to a mill and enjoy overkill... cut out a section that is about 0.6" long and will allow air soft rounds to drop in and roll out either end of the ball point pen body without jamming.  

Drill a hole in the center of the pen body opposite of the hole you just cut (see pictures for reference.) The hole should be slightly smaller than the diameter of your dc motor drive shaft (because that is where the barrel will be mounted.

Note: You will notice in one of the pictures that I have cut out a small section from another pen body to reinforce the section of the barrel we cut out.  In retrospect I feel this is unnecessary.

Step 3: Make the Magnetic Trip Switch...

Making the magnetic trip switch...

I thought the best switch to use for this would be a aluminum foil magnetic switch that I dreamed up on the fly. This will essentially allow you to put a spacer in between the switch that when pulled out (e.g. with fishing line) will allow the magnet to stick to the paperclip and close the circuit.  Ignore the binder clips; they turned out to be unnecessary.

Take a small piece of paper and tape a paper clip on one end.

Tape on a piece of aluminum foil over the paperclip.  Leave enough aluminum foil to wrap around on both sides of the paper. Be sure to leave as much of the aluminum foil exposed as possible.  You will need one side on the bottom that has no tape on it so we can connect it into the circuit later on. 

Tape on a second piece of aluminum foil on the other end of the piece of paper being sure to leave plenty of space in between the two pieces of aluminum foil. Make sure that when folded in half that the aluminum foil on the non-paperclip side is not covered with tape so they can make contact with each other to complete the circuit. 

Tape a magnet on to the non-paperclip side so that when the switch is folded in half the magnet will sandwich the two aluminum foil covered sides together.

Step 4: Build Your Paperclip Pen Hopper and Add a Timing Bump Switch...

Building the hopper...

The hopper is another disposable ball point pen with the ballpoint pen and ink well assembly taken out. I used the cap to prevent ammo from bouncing out.

Building and attaching bump switch timer to the barrel...

1. Take a large paperclip and bend the small "U" away from the large "U" into what looks like a number 8.

2. Bend the paperclip into an "L" shape as pictured. 

3. Tape the small "U" to the top of our barrel with your favorite tape of choice. Masking tape works well for this.

Building the hopper holder...

1. Take one large paperclip and and straighten.  

2. Than using needle nose pliers bend into a u shape.  Leave plenty of length on the uprights of your "U".  They will stick in to the corrugated cardboard and allow us to position the hopper right where it needs to be.  The extra length also allows it to be positioned far enough away so that the barrel will not swing into our hopper holder. 

3. Take another large paperclip and straighten.

4. Using the needle nose pliers wrap one end around the corner of the "U" we just made.  Wrap enough time so that the two paperclips will not shift relative to each other. 

5. Bend the paperclip we just wrapped around our "U" into an "L" shape. 

6. Take another large paperclip and straighten.

7. Wrap around the "L" we just made to extend the length of the "L" and give you enough to wrap around the hopper enough times to secure it tightly.

Step 5: Build the Motor Platform and Circuits...

Building the motor platform and awesome aluminum double-sided tape circuits...

The motor platform is built from two pieces of corrugated cardboard.  I secured the motor to the cardboard by saving the mounting screws from the motor that I harvested from a broken portable DVD player. After putting a piece of tape on the top of the cardboard to prevent tare-through I poked three small holes that corresponded with the drive shaft and two motor mounting holes and screwed the motor on with the above mentioned screws.

To create the circuit that would connect the battery and motor to the magnetic trip switch I used a marker and drew out the circuit on the cardboard first. Then I laid double sided tape over the lines I drew.  Then I placed a piece of aluminum foil over the double sided tape so no double sided tape was left uncovered.  Using an exacto knife I cut out the aluminum foil that was not connected to the double sided tape.  Then I covered the circuit with clear tape to prevent accidental shorts. I DID NOT cover the ends of the circuit where I needed to make connections.

note: I really like the aluminum foil / double-sided tape combination for a circuit.  While I am sure it's been done.  It was new to me, and I was proud that I thought of it on my own. 

I mounted the motor / 9 volt battery assembly, and the magnetic trip switch with the double sided tape.  I used clear tape to secure all the motor leads an other connections down.  I also used tape to secure the cardboard around the 9 volt battery.

Note: While you are working with the magnetic trip switch you should place a piece of  cardboard in between so that the switch can't close the circuit.

Step 6: Assembly and Deployment...

Assemble the machine!

1. Press fit barrel onto drive shaft of motor.

2. Insert paperclip hopper holder into the side of corrugated cardboard. 

3. Adjust paperclip hopper holder until the hopper rest in the center of your barrel opening.

4. Adjust hopper to hang low enough that when it's filled with air soft rounds they do not roll out of the barrel. But when the bump switch rotates under the hopper holder it slightly bumps the hopper up allowing a round to roll out the barrel.

Find something to hide your centrifugal booby trap machine gun in...

I know the box I used is lame.  I simply ran out of time before I needed to post this for the weekly challenge. You can use anything that will cover the device and have a small piece of cardboard and fishing line attached to it so that you can place the cardboard in between the magnetic trip switch.  

Set the trap and enjoy...

When the item is removed by unsuspecting victim the cardboard will be removed and the magnetic trip switch will close.  Mwha ha ha! Let the airsoft madness begin. Okay... mine wasn't really powerful enough to make the air soft round hurt or cause that much madness. But the air soft pellets are really annoying to pick up.  And actually I am glad they didn't hurt, booby traps that hurt are not fun.

Thanks again for reading and good hunting!



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    15 Discussions


    4 years ago

    cool project :). can you make more airsoft traps?


    4 years ago

    Made something similar once using the little broadcaster out if a seed spreader and a motor from a rc car it used a funnel for a hopper with the trigger being at the base of the hopper so when the trip wire was pulled it both turned on the motor and dumped the bbs onto the spinning seed spreader it would cut through a cardboard box if you loaded it with pop corn kernels

    Big Baneser

    5 years ago

    I have a birthday card with a spinning candle it will work good for this purpose


    6 years ago

    You are brilyent


    7 years ago on Introduction

    I just finished building one from an old rechargeable air mattress pump, and reusing some of the shell it came out looking nice. I loaded it up in a box and tapped the wires together, and in that split second, it emptied the hopper. I used a 3rd pen cartage to hold the center wire up and model glue to fuse the hopper to the barrel, and the guard to the shell. Now to make a dead-man switch muh wah ha ha ha

    3 replies

    I apparently set something on top of mine, making the hopper jiggle, so I put a little more model glue to hold it into place, and being the inpatient person that I am, I started it up, and ended up with "cobwebs" shooting from it. something that I am going to have to try for halloween affects :D

    I'm going to tinker with it, i I remember correctly, mythbusters made something like this, and had it so that it would shoot out of one spot... now to just drink aa 2l of soda to harvest it's bottle


    7 years ago on Introduction

    hey its good but you need more clear pics and more details because i cant figure out how it launches the balls....


    7 years ago on Introduction

    Sorry, i might sound a bit snotty here, but it seems my physics lessons have finally come in handy. Centrifugal force is not real, you may be referring to centripetal force, is the force that's causing the pellets to shoot out of the pen. The pellets want to move in a constant speed straight line, but the barrel of your pen is pushing against the pellet while it stays in place when the pen rotates, causing the pellet to fly out of the pen. Just like those amusement park rides where the ride spins around really fast, no force is pushing you out, its just the force of the wall behind you that is constantly pushing you in a circular direction, your actually accelerating toward the center of the circle. This is referred to from newtons first law, objects in motion or rest want to stay in motion or stay at rest unless acted upon by an outside force. Yay physics!

    1 reply

    Reply 7 years ago on Introduction

    Umm... Not to be snooty either. Yes you are right, technically speaking centrifugal force is a fictitious force. However it is very common to use the term centrifugal force when describing forces applied on an object from it's frame of reference.

    Or in other words theoretically speaking the air soft projectile from it's frame of reference is experiencing a centrifugal force, or a force causing it to move away from the axis of rotation of the machine gun motor. Centripetal forces are used to describe forces pulling an object towards the center of rotation.

    I appreciate the physics theory here. But it has become quite standard to reference centrifugal force in different mechanical systems that utilize rotational forces to fling an object outwards. For example a centrifuge is a device that references centrifugal force in the name, or describing how the system works.

    Here's an example of another machine gun invented quite some time ago that references centrifugal force.

    Regardless of how fictitious centrifugal force is in physics theory, according to laymen's terms it exists enough to be a common way to reference mechanical systems that utilize rotational forces to fling object outwards. The centrifugal machine gun is intended to fling projectiles outwards, not pull them inwards. Otherwise I would name it a centripetal machine gun. But thanks for being thorough.

    relly coll but need beter pictures when you add the moter becouse i cant figure out how it lunches the pellets