Using a clear tube from a FLUM vape device, and 2x 12x40mm compression springs, 4x M3x10 Screws w/Nuts, 1.75mm Filament as a dart rest/block, and 10-32 allthread. This device relies on the flexible/compliant nature of plastic, in order to provide a trigger mechanism, and is optimized for PLA.

There is ample room between the piston base and the tube end cap, to allow for standard nerf springs to be used, I think. I don't have one to check with..

Print 2x of the Tube Cover STL if you do not use the plated files in the PrintedParts.stl

Here's a video of my using it, since apparently Printables doesn't support videos in the model gallery - https://imgur.com/v112oeP.mp4