Mech boxes (gearboxes) are designed to fit into different shapes and sizes of aegs. Some are very strong, others have weaknesses. All have their quirks. Some parts are interchangable others are not. No mechbox is interchangable between different guns (other than maybe the MP5 series, never tried to change the type). So you can't take a version 3 and put it into an Armalite gun (M16/M4, etc.) It physically wouldn't fit and there's no way to make it fit without the mechbox sticking out on the outside of the gun in spots.
Version 2 and 3 are the most common due to the type of guns they are found in. Most commonly upgraded internal parts are interchangable between these two, however some parts are mech box specific (like the spring guide and cylinder head) while other parts are specific to the gun.
Here's a breakdown of the most commonly found mechboxes. There are others, but these are the most common. STAR/Aries mechboxes are plastic (though very strong), but are fairly rare and I do not know if a normal gearbox would fit into STAR/Aries guns. I would think so, but am not sure.
Version 1 Mechbox: FA-MAS series
Version 2 Mechbox: All Armalites ie: CAR15, M16A1, M16VN, M16A2, M4A1, M4A1 RIS, SR-16, XM177e2, M733, M4-S. All G3/G33/CA33 series. ie: G3A3, G3A4, G3 SG-1, MC-51, G3 SAS. Most MP5 series. ie: MP5A4, MP5A5, MP5SD5, MP5SD6, MP5RAS, MP5-J
Version 2 Mechbox ICS - ICS M4 - split gearbox, will not fit any other make of body and the ICS body will not take a normal V2 mechbox. most internals parts are also specific to this mechbox, as in you have to use ICS parts for many internal parts, normal AEG parts won't work.
Version 3 Mechbox: G36 series, SIG 55x Series, AK series AUG series, UMP, MP5K, MP5K-PDW
Version 4 Mechbox: PSG-1
Version 5 Mechbox: UZI
Version 6 Mechbox: M1A1, P90 Series
Version 7 Mechbox: M14
Version 8 Mechbox: Type 89
Systema PTW's are in a league of their own. Nothing on or in them is compatible with a regular AEG. Aftermarket accessories don't count.
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Last edited by Darklen; December 18th, 2008 at 00:07..