As promised; an update on the CA shield: it works exactly as I thought.
THE BLASTER IS A SERIES OF TUBES.
The rear handle operates a large gear mechanism that pops the shield into firing position, and a Snapfire type plunger assembly is used to propel the darts.
Tiny plunger stroke, but wide tube.
The trigger also operates a 5-pronged rotation gear, which alternates air flow between the two tubes (which then uses the air restrictors to select the firing barrel, like a Rough Cut or Triad). The fifth gear pushes a tab on the gear system, releasing the stored potential energy to fold everything back into the normal shield shape.
Note that extra plastic on the rotation gear.
Obvious experiments include a stronger extension spring for the plunger assembly - unlike the Snapfire, you're using all four fingers to pull the trigger, allowing more force to be applied without your fingers tiring out. Efforts to shorten the tubing may also be of interest. In the end, though, I'm still afraid that complete replacement of the internals is required to make this shield really useful in a Nerf War.