SpudFiles

I had this idea to "optimize" simple pen-blowguns which would be to simply add a piston valve so your breath is at it's full pressure before it blows the pellet. This is opposed to the gradual build up in pressure you get by just blowing, which may lead to the pellet leaving the barrel before you are blowing at maximum strength. Is this idea feasible at all? Or would this be weaker than just blowing?

I don't intend for this to have any kind of high power, just something that would be easy to make, slightly more powerful than just blowing into a barrel, and what won't leave me light-headed.

Use a detente, a very weak one considering the low pressure(probably less than 1 psi )
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Force = Pressure x Area.

Pen tubes are tiny, so they have a small area, so for a large force, you want high pressure.

The human body can supply something in the region of 3-4 psi at best, so for a blowgun that is a limiting factor, which is why you want a bigger area and therefore larger barrel diameter is the way to go.