In the scene where he is cleaning up King Willie's skull: he places the skull up on the display spike and slowly withdraws his hand. Now, when he does this, look closely at the area under his wrist - you can see a rectangular slit where the dart is fired.
That seems to be the only reasonable explanation.
The slot is big enough for the dart.
The Netgun is excluded. In addition, there are 5 spikes on it, not four.
Thanks for your help and cooperation!