Gearbox's are the main part to a airsoft gun, well an AEG i could spend hours explaining all of how each gearbox works, version 2 and onward theres alot, but im going to let one of my more favorite youtube people do so, airsoftgi a airsoft store who also keeps open and know with there youtube!, i order from them, theres also evike, airsoft megastore, airsoft california, blahblahblah you get the drift? but below is the video it shows all you need to know about AEG's i give all the credit towards Airsoftgi.com

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