You should be able to double that pressure within the limits of the fittings you're using which will give you a lot more power. Roughly twice as much, actually
Consider using a
2 inch sprinkler valve that will give you much more flow compared to your current setup.
For an air chamber, a section of PVC pipe roughly the same volume as the barrel will be more than adequate and not have the same flow restrictions.
For the ultimate performance without too much extra budget, consider a burst disk setup:
Your chamber and and barrel are separated by a union in which you have a disk of frangible material (like for example aluminum foil) sealing off the chamber. In order to fire, you pressurize the chamber until the pressure become too great for the disk to contain it causing it to rupture. The burst disk is the ultimate "valve", with virtually zero opening time and maximum flow. The disadvantage of this setup is a small delay between starting to feed air and the valve bursting but for a bait cannon this should not be an issue.
Also don't neglect projectile design, a simple cylinder will not travel as far as a projectile made with some attention to aerodynamics. Here's a possibility, assuming you're freezing it, a round nosed projectile with a cardboard "tail" that will stabilize it by drag:
Also make it
Pykrete style by mixing in some wood pulp to make it more likely to survive the acceleration forces in the barrel.
