Ok, So while this is my first attempt at something on this level, I think I have a good idea, so tell me in anything which could be improved upon. I will be attempting to build it this summer, just for reference. Anyway, here is a basic overview of how it would work, because it is fairly complex. (note that as of the time of this post, I have already made and tested a lego model of the clip system, and it works with very few kinks, except nerf darts being too long.)
First let's start with the Magazine assembly (fifth picture). It has three main parts (as shown in the first picture). These are the Follower (top), Main rod (middle), and ejection lock (bottom). The follower acts like with any other En Bloc, moving the darts up. The Follower is being powered by a spring under the main rod. the clip itself has groves which allow the follower to use it as a track. Anyway, when the follower moves up, It rotates the Main rod, due to connecting rods. This in turn moves the ejection lock, which releases the clip ejector when the follower is at max height. This is accomplished by the use of groves cut into the side of the ejector, like Lincoln logs, that hold it in place until the clip is empty.
The other major part is gas piston (Shown with its various parts in pictures 2, 3, and 4. It is essentially a single acting Pneumatic cylinder with a built in quick release valve. This means that it is a pneumatic piston, with a spring return, and a button to release all the air down the barrel. This works by having the pressurized air push the the bolt, with quick release, into place. The trigger just sits in the grove until the bolt is closed. when the trigger is puled back, it unlocks the seal on the valve, and all the air used to move the piston is used to move the dart down the barrel at high speed. the spring then returns the bolt to its original place, letting the next dart move up, and reseting the whole system.