[Pendulum^-1]

Is it possible? Yes. A number of teams did this in 2008. I distinctly recall Blue Cheese, with their robot Blue Cheddar, at VCU:

http://www.youtube.com/watch?v=c1LK0JnMPFI
http://www.youtube.com/watch?v=KMZaZiNkDFI

There are probably other videos with better launch characteristics out there. Quite impressive. And that ball was 41" diameter, and 7 pounds. A real crowd pleaser!

As I recall, they did not latch the launcher. They did it all with improving the speed of the air flow into the pistons by reducing air drag. That, and they had four parallel pistons, about 1" diameter, maybe 8" long, pushing their shooter/sling. (I can't remember the precise dimensions for sure, but I did talk to them a bit during Saturday morning at VCU, observing and admiring their system.)

Ways to improve air speed into the pistons:

1) Having air reservoirs/tanks on the low side. A main source of air flow drag in pneumatics is the regulator. If the air is in working (low pressure) side tanks, that is no longer a source of drag for the air flow into the piston. Keep in mind that means that those tanks are only at 60 psi, so they don't store as much energy. You'll still need high side tanks.

2) Have a dedicated supply line for each piston. Tank to solenoid to piston intake.

3) Piston exhaust is open, with even the brass fitting removed. Be sure to use double acting pistons: single acting pistons have springs which will reduce your power/speed. With this setup, you'll have to have an alternative way to retract the shooter. Gravity may be sufficient. Pushing the ball onto the shooter may be sufficient to retract the shooter.

Latches are necessarily complicated mechanisms. More complexity means more potential failure points. If you don't need a latch, avoid using it.