I just made my bad ass pen dart gun. I have like 15 more cans of air to play with too! I spent a good amount of my work day playing trying to perfect my darts.
You could increase velocity and accuracy by putting the dart in the opposite side of the pen tube (so it travels the length of the tube before being expelled).
The pen's butt cap needs to be on the opposite end and it needs to be part of the dart. The butt cap seals the tube off so pressure will build in the tube. We tried filing an eraser down so it could slide inside the pen, but it did not work so well. There is also another video where you make the same dart but instead of using a can of air, you load a spit ball into the opposite end and use the ink stick to quickly plunge the spitball up the tube towards the dart, which will increase the pressure until the dart flys away. That method works, but not as good as some duster. Damn, i am pretty bored today!