How about the middle ground solution:

I seriously doubt that an AEG style GL would provide sufficient power, your best bet would probably be self containted gas charged shells. The ideal one would probably be those recently posted Mad Max shotgun shells (5 8mm BBs each, right? Could probably mod it to shoot slug or a lot more 6mm). They're smaller than the 27mm moscarts (so closer to the real 20mm) but also pack quite a bit of power. Next best, external gas. Lord knows there's enough room in the OICW to cram a tank somewhere :p. Still, this system is rather overly complicated but at least an example already exists in the M1100.
How about an electronic load/fire/extraction system for selfcontainted shells? It doesn't seem too difficult to do, and is easy to intergrate with the rifle section's electronics. You'd basically just need a AEG style mechanism, only instead of drawing a piston, it'd move the bolt to load/extract.

Also, just on accurate replication, the battery would have to be in the rifle, not the GL. Afterall, the rifle section can function standalone, while the GL can't.

I like it :D it would work just like that M4 shell-ejecting gun, only so much cooler. Only catch is making sure you pick up your shotshells afterwards (too expensive to be disposable)

it looks great

For the select fire option, start off by making a system that would fire both rifles at once. That would allow a working setup. As for the carabine, which would be aEg (note the emphasis), simply make an electronic shut off switch between the battery and the AEG motor. So when you clip the upper section on to the AEG, the top will fire (since the setup allows miscellaneous fire) but do to the cut off, the carabine won't fire. The same system would be applied to the top part.
Some parallel wiring here and there, basic switch system (paper clip and plate :P ) and you got yourself a fire select weapon.

**note danger of shock when in contact with water. Remember, electronic tape & duck tape are your best friends. **

Duck tape is already my best friend. :cheers: