Launch It: The Pneumatic Potato Pistol.

Step 3 Construction.

The nozzle of your syringe (do I mean nozzle? I bet there's a medical word, but you know what I mean) is probably longer than the internal diameter of your pen. Trim it down to give plenty of clearance to allow air through.

Lay the barrel across the top of the syringe, with the stump of the nozzle in the hole you drilled. Hot-glue (or epoxy) the barrel in place, making sure there is an air-tight seal all round the point where the nozzle enters the barrel.

Let the glue cool fully before using the gun. This can take longer than you think, especially if your layers are thick.