first of all what make is your ak, if your gearbox is a plastic gearbox, made by a cheap clearsoft manufacurer i dont think it would be worth upgrading. 310 fps is fine for outdoor games also, all the high end AEGs right now and even majority of cheap aegs are based off the marui design, everything should be marui compatible unless you have some cheap crappy gun made from some unknown manufacturer.
but however i think springs should fit? not sure, anyway m120 is about 320 to 350 fps right now, also having a longer inner barrel and other factors also affect FPS,
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