Replaced PDI W-Hold rubber bucking with Prometheus purple bucking. When chrono-ed, got results anywhere from 405 to 425 FPS with 150% PDI spring. I am surprised I could get such a big increase in FPS from 150% spring...
Replaced VFC nozzle with much more tapered Lonex nozzle and cut one loop off from the tappet return spring. When I pressed the hopup against the open gearbox I could see tappet plate goes 2mm back inside the gearbox when the nozzle seals with the bucking lips, so it was supposed to be a good seal. When I assembled everything together and chronoed I got 394 - 399 FPS consistently... which is still inconsistent. I guess I need a slightly longer nozzle. Not sure what else to do at this point. VFC nozzle has larger inner diameter then Lonex one and it is not round (like on the pictures below) but the VFC one is 0.3 mm shorter. There are no leaks when I do cylinder to barrel airleak test.
VFC nozzle:
Lonex nozzle:
