For t-shirt shooting since you only need to turn the air supply on and off I am a fan of these valves.
http://www.airbagit.com/Brass-Air-Va...irvalve-06.htm They do make them smaller too. They are cheap, completely rebuild-able, fast acting and are very simple and reliable. They do need a bit of power to operate, around 4 amps per valve, IIRC. They have smaller versions available too.
We used them in testing for our abandoned T-shirt shooter project and we were able to control the distance of the by the amount of time the valves were opened.
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