I just pulled up the schematic for the FX Dynamic and am shocked at what little information is available. Normally FX is very good at providing all the specifications and details needed to rebuild their airguns. Wonder what happened with this unit.
Sourcing O-ring can get to be an interesting journey. At times I have had to order some O-rings from one place and others from a second and then look to a third vendor for those missing units. Believe it or not, Amazon has gotten me out of a couple of tight spots with odd sizes for some of my Chinese guns. Go figure.