The Safe "Exploding Pen" (Prank Pen Cap Gun)

Introduction: The Safe "Exploding Pen" (Prank Pen Cap Gun)

About: Illegitimi non carborundum.
Thank you to the voters and editors who helped this instructable make it as one of the 2nd place finishers in the 2013 April Fools' Contest.  Please build/adapt this for your year-round pranking applications!

NOTE:  This instructable covers a harmless prank device, not a weapon or a destructive device.  It utilizes toy strip or ring caps used for children's cap guns, found in many toy and grocery stores.

The "exploding pen" is a common item found in novelty and magic stores.  These ugly prank devices somewhat resemble a crudely molded pen, and contain a small mousetrap-like device that sets off a toy cap when the pen's cap is removed.  As a kid, I was very excited to acquire one, and quickly improved it by housing the initiating device inside a real pen. After pranking a couple of my friends with it, one of my friends was struck on the face by a tiny spark.  He was not injured in any way, but this incident caused me to shelve my pen for safety reasons.

The primary design problem with the original "exploding pen" is that the detonation of the toy cap is relatively uncontained, allowing a shower of tiny and usually harmless sparks to fly in all directions.  I have included a picture of my old pen firing.  The odds are low, but occasionally, these sparks can connect with the target of your prank and cause minor pain or even a freak accident like this one.

My redesign has addressed these safety concerns on two fronts.  First, by enclosing the toy cap and initiating device inside the body of a pen, almost all sparks are contained.  Second, simple body mechanics make it difficult and unnatural to click a ballpoint pen while pointing it at your face. 

I decided to modernize the design by housing it in the ubiquitous Bic Clic Stic, often available for free as a disposable advertising medium.  I give you the improved, safe "exploding pen".

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Step 1: Tools and Materials.

You will need:
  • a "Bic Clic Stic" pen (commonly given out by businesses for free as advertising or easily found online)
  • a short section of 1/4" drip irrigation tubing (or improvise with any other cylinder that can fit inside the pen body)
  • 1x 5/32" pop rivet
  • any brand of lighter containing a piezoelectric igniter (cigarette-type lighter, not a BBQ-style lighter as these usually contain much larger piezoelectric igniters)
  • toy caps

Other materials and tools that I found useful:
  • super glue
  • 1/8" (3.18 mm) drift punch
  • razor saw
  • mill bastard file or swiss files
  • drill
  • 13/64" (5.16mm) drill bit
  • 7/32" (5.56 mm) drill bit
  • small hammer
Metric conversions are provided so you can approximate dimensions with whatever you have on hand.

Step 2: Dissassemble the Pen and Lighter.

You will only use the highlighted parts for this project.

Step 3: Build the Cap Retainer.

File down the outside diameter of the pop rivet collar until it can freely slide in and out of the pen body (reduce to approximately 1/4").  I used my drill as an improvised lathe.  Using downward pressure, unseat the collar from the rivet pin and slide it off.  Check the fit of the collar with a toy cap and reduce the diameter with a file if necessary.

Next, cut a short segment of drip irrigation tubing to the same height as the inner cylinder of the rivet collar.  Partially drill out the interior diameter of the drip irrigation tubing with the 13/64" bit (to at least a 3/16" depth).  The unenlarged portion of the drip tubing interior should friction fit around the bottom of the inner cylinder of the rivet collar, and the enlarged portion faces the top to accept a cap.

The end result is an assembly that can easily slide in an out of the pen body to facilitate a cap reload.

Step 4: Prepare the Piezoelectric Igniter.

Cut the piezoelectric igniter at the bottom of the indentations as pictured. 

Point in a safe direction and click it, and a small metal cap lodged in the central channel of the igniter should fly out.  Ream out the end of the channel until the captive reset spring inside can slide free.

Carefully file the cut end of the igniter down until it matches the third picture.

File the corners of the igniter down until it can freely slide in and out of the pen body.

Step 5: Build the Initiator.

Use a drift punch or similar object to press firmly on the center of the pen plunger.  This will release all of the components shown in the second photo. 

Cut the plunger at the middle indentation as shown in the third picture, and click it back into the plunger shell with a small amount of super glue. 

As in the fourth photo, file down the corners of the ingiter plunger until it fights tightly in the pen plunger, then press together with super glue.

Line up the pen clip tube with the igniter and plunger, and mark and cut the tube of the pen clip flush with the igniter body.  Compare to an intact pen to ensure that the pen plunger remains at the correct depth.  Save the portion that you cut off to use in the final pen assembly.

Drill out the interior diameter of the pen clip tube to 7/32", or until the pen plunger can pass freely through to complete the initiator.

Step 6: Final Fit.

Stack the initiator on top of the cap retainer. Measure the total length of these two parts, and ram the excess pen clip tube (trimmed off in the previous step) to this depth inside the pen body.  I tapped it into place with a nylon rod and a small hammer.  Friction should be enough, but sparingly applied super glue can be used to lock this interior collar in place.  This will serve as a stop for the cap retainer, holding it at the proper depth to be struck by the initiator.

Step 7: Load and Test.

Load a cap into the cap retainer, place it inside the pen body, and slide the initiator into place.  Test fire, and adjust the interior collar depth if necessary.  Apply judgement in use of this pen - not recommended for use on or around those with heart conditions, advanced age, or certain categories of PTSD.

The design of the cap retainer can be endlessly modified based on the materials you have available - the final picture is of some of the past designs that I have experimented with. My design goal was to create a mechanism that does not significantly change the weight or shift the balance of the pen, and also safely allowing the expanding gasses to vent out of the tip of the pen, thereby containing most of the sparks and avoiding excessive fouling to the striker mechanism.

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    4 Discussions


    6 years ago on Introduction

    I looked at the article for the kid, and that is bad. I will try to make an instruct able on a water squirting one later


    7 years ago on Introduction

    This is so cool! I don't have the smarts to make it though, lol.


    Reply 7 years ago on Introduction

    Thanks! It's really just about finding or modifying small components that fit - that's where I have the most fun when I make stuff.


    Reply 7 years ago on Introduction

    That is exactly what I think every time I see an instructable involving sewing, welding, soldering, or the million other things I can't do well on here, lol.