This is something I've been working on for the past few weeks, finally feels like it's gotten to the point where it's good enough to post. This is a variation on the "schnepper" devices that were first seen in the 17th century renaissance europe, adapted to the plastic (PET) parts which tend to handle compression/expansion forces better than sheer forces. Unlike some of the former devices I have posted, this requires no tools to load or fire. I also shrunk the darts quite a bit to better deal with wind or other outdoor disturbances, and they now have embedded metal rods to add weight (the green one is around 100 grain, the violet one around 200 grain).
There's a video of the initial prototype being fired indoors here:
This one lacks a lot of the features of the "final" version like the "stop/cover" (to stop the string from 'bouncing' over the bolt) and the "clip" to keep the bolt in place if the device is carried around loaded or aimed downward. But it exposes a lot of more of the inner workings of the trigger assembly.
I'll see if I can get some footage of the "final" device being fired outdoors soon, it is quite accurate with a light and crisp trigger pull, but so far not quite as hard hitting as some of the other devices I have built so far.