I get what your getting at, Pyle. Hell if it's GBB you could easily make it powerful enough to actually eject the spent case like a real round. One problem with ANY gbb sytem for the OICW design: Bothe the rifle and the grenade launcher operate off of the same trigger, so you'd have to set up some way of having one triger select-fire control both an AEG and a GBB.
One Idea, Two gearboxes, two AEGS... one standard 6mm the other firing a specilize 10-15mm soft slug. The Battery could be concealed in the space between the Grenade clip and the rifle, and have conectors running to both gearboxes. The trigger could simply swith witch AEG gearbox is being used. Not sure about how you'd cramm all that in there but I'm sure it's possible.
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