LOL, yup bigger better is on its way I was building the tiny one to learn lessons like the one I just sorted out. Now one more tiny to sort out geometry and efficiency, next a midsize and then I am going to go big. Stay tuned.

It fired! Not the best and it needs a revamp on the build but now I have something functioning to work with. Thank you all for the help. FWIW the floating O-ring was the special sauce. Thank you Gipetto. Now to fix some issues with the flanges, better sealing faces and work on optimizing 3D printed ...

Anatine, Yes they are 3D printed. I know 3D prints leak air but if you seal the ABS tube surfaces and put in rubber gasket or Silicone the hell out of it, it seals up nicely. You can see the 3D models in the link in the original post. https://cad.onshape.com/documents/73c1c66cd575fcedf1c33d95/w/a956...

Update: Longer piston does not help unfortunately. That rules out the piston jamming due to being too short and the pilot chamber being too big. I tried a air compressor relief valve thinking the pilot trigger was not venting fast enough. Again no dice. I am going to re-configure the pilot/fill area...

Cool that is easy to test. Ill make a longer piston and see if it is tipping, or too big a pilot chamber. Printer will be fired up in a hour and should have some test results tomorrow. Thanks!
Fingers crossed

I am building my first co-axial piston using a 3D printer and making a small one before I go big but things are just not working out. I can pressure up to 60-80 psi and hit the initiator valve but the piston does not move and dump air to the barrel. So here is my report: 1. Nice sealed piston was tr...