If you want to emulate a quick shutter press, you should probably do shoot_half and shoot_full in quick succession, and release them in reverse order when done.
e.g.
press "shoot_half"
press "shoot_full"
sleep 10
release "shoot_full"
release "shoot_half"
you could also try
click "shoot_full"
In general, when scripting button presses, it's a good idea to make them as close to what a human would do as possible.
Also note that some CHDK ports have issues with raw and/or overrides (including script overrides) if you press the shutter quickly.