Now a huge controversy in airsoft, AEG or GBB some prefer the realism of GBB, but others like the cheaper *in most cases*  but trustworthy AEG, what is your prefrence, mine has to go to AEG as well its simple to keep clean and running, and works like a charm, GBB are still a little "misty" to me... see what i did there? HAHAHAHA, but back to the subject, i dont have a true full understanding of GBB rifles and pistols, i have used some and truly enjoy the realism with the nice kick they deliver!, but for rifles i will always be AEG

An AEG Gearbox- the main mechanic that runs an AEG *besides the motor* *P.S this is a  version 2 gearbox which is very common in M4 *setting base as M4 as its most common* Variant rifles
Link- http://i60.photobucket.com/albums/h24/ajpadre/CA_M15A4_CQB_Gearbox.jpg