Blowguns are made for cleaning surfaces with compressed air, but since they have a comfortable grip and a built-in valve that is easily opened by hand, they're commonly used as pilot valves in pneumatic cannons.
Blowguns have rather limited flow - enough to trigger a small valve, on the order of a 1" sprinkler or slightly bigger, but will not provide enough flow to trigger large valves. For larger valves, it is necessary to use them to trigger a small valve, which then acts as a pilot valve for the main valve. Flow can be increased substantially by modding the valve.
Most blowguns come with a nozzle, and this should be removed since they (almost always) reduce flow further. Most blowguns will only work in one direction, and will leak if pressurized in reverse.
In BB machine guns, a blowgun can be used as the primary valve since these guns need less airflow.