Double Chambered Air Cannon
Here's an air cannon that uses two chambers instead of one. Air is compressed into each chamber and then released into the barrel via a sprinkler solenoid. I'm skeptical that the double chamber actually helps because when I built one of these the sprinkler valve was pretty much the bottleneck of the whole system. Putting more volume of air behind it doesn't really help after a certain point, it's how fast you can get that air into the barrel that matters. But then again, I'm no scientist, so if you'd like to weigh in with your opinions please do so in the comments.