piston designs

[f.c]

i have looked around a fair bit and have decided to try my hand on making a piston inside of a 25 mm t section. piloted by a blowgun i have already got an idea but it includes filling up both the chamber and the pilot area.

is there any suggestion of deigns from anyone would be so unbelievably helpful. by the way im trying to make the gun in an over under design.

[meatballs]

one easy solution would be to use a sprinkler valve, although it looks like you want a piston, and i'm sure you've considered a sprinkler valve already. basically, just do some research, figure out your design, and build it. at that small of a size it wouldn't be too expensive to mess up a couple times, and you learn a lot by messing up.
and to solve the problem of filling the pilot chamber and the pressure chamber separately, just add a small equalization hole in the piston between the pilot and main chambers. this way you can fill the whole thing from behind the piston. or if there's a spring behind your piston, then you can fill from in the main chamber side and the pressure will equalize behind the piston.