Whilst reading the responses to the topic, I came up with an idea. You take two different sizes of PVC and a torsion spring. You would place the spring inside of the smaller diameter PVC, then, attach one of the spring "arms" to a solid base inside the PVC. Next, attach the other "arm" to a dart friendly loader, (basically something that pushes the darts along in the clip without them jamming) and place the loader arm in the space inbetween the two sizes of PVC. Here is a diagram...
The darts are terribly out of proportion, but you get the general idea. Also, the spring would have to be weak enough to not squish the darts, yet strong enough to push them ALL the way up to the breech. It's a conceivable idea, I might actually try it if I can find a spring. -iMod
In addition to the above idea by iMod1, you could also add a crank to turn the drum mag clip. My idea was more for a prototype/spring failure backup king of thing.