Nice cannon! Clean build.
Some comments:
-You could take that T-piece with the pressure gage out of your pilot volume and tap the gage in something else. Like in the chamber, or maybe even right into the sprinkler. This will increase performance a little.
-How well does your bolt seal? Did you add an o-ring or something (even duct tape) to seal it?
-About the suppressor:
It works, but it could be more efficient. Besides increasing the volume, you could also build a suppressor working in another way.
Make it vent air out to the outside. In order to slow the air vented out, add baffles. May create a short hiss if the baffles are not obstructing enough, but surely does the best job suppressing the bang.
When going for increasing volume, you could also make an expansion chamber around the barrel (Like a coaxial launcher) but in this case, the chamber is part of the suppressor.
Also, if you fill the suppressor with steel wool you will get some improvement.
Mainly for combustions, to cool the gases.
Does work like baffles, obstructing the air though.
but i would still like to attempt a piston valve cannon. 
