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I think I used 30%, maybe 60%. You don't want it TOO hollow(10% way too little), because it has to handle the recoil+weight of the gun. If you have the plastic to spare, make it as tough as you can, maybe 70-80% infill(I think I used 70%). Mine is pretty solid, fired over 2k rounds, carried it by the foregrip, used the foregrip as a bench rest, and still hasn't budged or shown signs of cracking.