Just posted my thoughts and findings regarding Predicting Pump-Action Water Gun / Water Blaster Performance. In short, it appears one can use either pump stroke length or, if available, pump volume to reasonably estimate expected output for most pump-action water blasters. It is unclear how this scales up for larger, syringe-type water blasters due to lack of available measurements for those type of water blasters. Follow the link above to read more specifics!
The Beauty of Water Blasters, Water Fights, and More