Kudos on using the 5-way valve. It's not to its full potential though. It's technically just a 4-way as it is now. Let me make a suggestion.
As long as your valve will work in reverse directions, you can eliminate the manual fuel valve by arranging the port configuration as shown. Your fuel will fill the meter pipe while air is filling the chamber, and in the other position the meter will dump while air is pressurizing the piston. After the shot, it will be in position 1 already, so air will purge the chamber automatically.
The only thing I can think of that may get in the way a little bit is that some of the fuel may escape while the piston is seating. With a little tweaking of the meter pressure, it's probably of no real concern.
Also, you can get rid of the little ball valve at the piston by replacing it with a check valve.
Awesome cannon. I hope this helps to improve it.
