This diagram shows the rough dimensions of the "3 by 1.5" gun that I built first.  The compression chamber was made from 3" PVC and the barrel from 1.5" PVC.

This diagram identifies the 3 chambers that are referred to in this theory of operation page.  Note that the compression chamber is the area around the barrel, sort of like a cylindrical volume with a hole down the middle (where the barrel is).

Here you see the areas where glue is used, as well as the critical sealing areas.  As seen in the pictures, the sealing surfaces of the back end of the barrel and the circumferential face of the cleanout plug must be very smooth and uniform or the gun won't fire.

Now let's see how the gun actually works.  In the first phase, the blowgun is pressed up against the small opening drilled into the back of the cleanout plug, and its trigger depressed.  The high-pressure air is delivered into the cleanout plug cavity and as the cavity is quickly pressurized, air also begins to leak past the outer edges of the diaphragm and pressurizes the much larger compression chamber cavity.  During this phase, the diaphragm actually bows a bit, but the bowing exaggerated in this picture for effect.  This phase lasts maybe 3-5 seconds, depending on the size of the compression chamber and the capacity of the compressor to deliver air.  The operator usually determines the end of this phase "by ear", or when the sound of hissing, flowing air fades away.  Some guns will "whine" ominously during this compression phase.

This illustration just shows the relative states of pressure at the end of the compression phase.  At the end of the compression phase, the gun is still held firmly against the cleanout plug and its trigger depressed but no air is flowing.  The pressure in the air line, the cleanout plug cavity and the compression chamber is all equal.  In most instances, gun operators don't take the pressurization quite all the way to this equal pressure state before firing.  Once the sound of flowing air fades away, the gun is usually fired.

Phase 2 is the depressurization firing sequence.  This phase starts when the trigger on the blow gun is released and the gun simultaneously pulled away from the cleanout plug aperture.  When this happens, the cavity behind the cleanout plug depressurizes out of the aperture.

Once the cleanout plug aperture has depressurized, the diaphragm cavitates backward slightly (exaggerated in this diagram) does two things.  It blocks compression chamber air from escaping out of the cleanout plug aperture as the diaphragm moves back hard against the cleanout plug; and it allows the compression chamber air to very rapidly escape up the barrel, expelling the munition (potato).

As the compression chamber air rapidly escapes up the barrel, it (of course) expels the potato very quickly.  It also very often will make the gun emit a sharp "barking" noise and if there is enough moisture in the compressed air, a vapor wisp will be seen out of the barrel end of the gun after it has fired.