Not bad in concept. One thing I'd recommend including is how the 'sheath' around your breech opening be extended to the rearmost of the magazine well. That allows a minimization of dead space in your cylinder. Also, to minimize dead space behind your dart in your barrel, have the breech opening as close as safely possible to the end of the barrel. Another thing is perhaps some way of locking your barrel forward to prevent any mishaps or unnecessary damage to your barrel's breech end.
I'm assuming the reason you opted to use a slit in the bottom of your barrel instead of the end of the brass tube is to prevent misfeeding from any damage that may incur on the barrel's end due to the pressure against the plunger/piston-spring assembly. I'm also assuming your magazine won't be curved, unless I missed something in homemade magazine technique during the past year or so I've been gone
![:)](http://nerfhaven.com/forums/public/style_emoticons/default/dry.gif)
I guess not bad. I still prefer pneumatic devices due to less moving mass...but that's personal preference. For a simple multi-shot spring launcher it has potential on paper. Let's see how your prototyping goes.
If this is going to be based visually of of the real FAMAS, you could very easily integrate a sort of slide-mechanism to make this thing pump-action
![:ph34r:](http://nerfhaven.com/forums/public/style_emoticons/default/ph34r.gif)