I know that blank firing guns are fine (don't think that you need a defece to have them either).
Yes you do, they are still a replica therefore covered by the VCRA.
Try buying one of those with Ukara. 
aka Stigroadie
AFRA
better by design
"Truth is a shining goddess, always veiled, always distant, never wholly approachable, but worthy of all the devotion of which the human spirit is capable. "
The Tornado airsoft grenades work on an air damper similar to the method Chomley described.
The main difference is that the sudden release of the Tornado dumps gas into the BB tubes.
Here's how the Tornado works:
http://www.airsoft-innovations.com/nadeart4.html
WRT the turned or cast choice: A turned chamber could be used in a cast body. Turning is the best way to make the working mechanism anyway.
My Tornado is horribly unreliable ,they can go off anything between 2 seconds and 30 seconds,and it has gone off in my face when I have gone to retrieve it .You wouldn't want a 12 bore blank firer to go off in your hand as you pick it up ,it's a sure recipe for losing fingers.
