Here is how to make a powerful compressed air blow gun with a pen.

Speed test (+-0.25% accuracy chronograph):
 – Average: 40 m/s = 131 fps = 89 mph
 – Max: 52 m/s = 170 fps = 116 mph

Kinetic energy (0.3g bullet):
 – Average: 0.24 J
 – Max: 0.4 J