Another thing to consider is reducing the amount of energy lost per ball. If your system compresses the ball a lot during a launch, more energy will be lost - which needs to be replaced.
Fuel weighs about 0.0707 kg, launched at 6 m/s will have an energy of 12.73 J
(
https://www.wolframalpha.com/input/?...elling+6+m%2Fs)
Shooting five of these per second is only 64W - ignoring other loses. Those other loses could be hampering your shooting rate.