I wonder if you're still not going to need at least a few metal parts such as the spring to power that? Still, even if that has to be procured separately it would be difficult to regulate the sale of springs.
The thumbhole "P90" stock is actually more interesting to me, it seems to alleviate the area that most other 3D printed stocks crack at.
edit: and just now I noticed the additional thumbnail views.. off for more mag p0rn.