This is an airsoft mod.
Top dead center (TDC) mod for VSR-10 Gspec and APS2 (Like the L96) and possibly many more rifles. The vsr specific part goes on the outer barrel of 30mm diameter, the aps2 to a 25mm barrel.
Glue an M4 nut in the tdcvsr.stl or tdcaps2.stl and glue them on the barrel above the hole of the TDC. Glue the tdcbottom.stl on top of it.
Insert and if needed glue the outside ring of the tdcspring.stl into the tdctop.stl. An M4 screw must be able to move freely up and down in this assembly, but with possibly no side to side movement.
You can use an M4 screw with a hexa head or just screw a nut tighly on any M4 screw (That's what I did.)
Experiment with the length of the screw. The spring must be pushed about half way with the hopup set. That will keep the top nice and tight.
No support needed to print, print the top upside down.
UPDATE: I have changed the top to have finer clicks, the spring to be a bit stronger and the bottom to be higher to protect and guide the top better.
For me a 12mm hexa screw was perfect length.
UPDATE2: on popular demand I did a video of how it works: