The syringe based poison arrow looked nice, but I haven't been satisfied with the amount of liquid it ejects when it hits the target. The pressure of the liquid that is squeezed through the thin needle is too high, it does not allow a quick enough discharge.
So I made my own "syringe" arrow!
I heat shrinked and glued on a piece of aluminum tube (7mm outer, 6mm inner diameter) onto one of the commercial bolts. Then I turned the tip from massive aluminum.
The tip is hollow until about 18mm from the tip (3mm longitudinal hole), at which point four 2mm holes are drilled through the tip. The rear part of the tip is turned down to fit precisely into the aluminum tube. It is now the plunger! A bit of epoxy at the back of the "plunger" makes it fitting 100%.
The four holes 18mm behind the tip are covered with a rubber band. If you fill the syringe with liquid, it does not leak.
The idea is that if the tip penetrates the flesh of an attacker, the rubber ring is shifted back by the tissue, opening the four holes. The inertia of the bolt pushes it forwards, pressing down on the plunger - and the poison is injected into the opponent.