For many, the workings of an AEG (or Automatic electric gun) are a mystery. This article is meant to clear that up, and give the reader enough know how to begin tuning and upgrading them.

See some more in depth tech here:
http://www.youtube.com/watch?v=QY9yA-zHMEE&list=PLA9841280D991F467&feature=plcp&context=C39ca8dbFDOEgsToPDskIlm2tdI8uH6ommMDnN8WI_
and here:
https://www.instructables.com/id/CheapFree-AEG-mods/

This article will be divided into three parts:
Intro to AEGs
How AEGs work
How to work on AEGs

Step 1: Intro to AEGs

electric airsoft guns can be put into into two catagories:
Tokyo Marui (TM) Based; MPEG (mid priced electrig gun) and AEG (automatic electric gun)
Non-tokyo Marui based; systema PTW, LPEG (low price, or low power, electric gun)
LPEGs have plastic internals, and often, funky gear systems, and Systema PTW is something I am simply not qualified to talk about so I will cover only TM based electrics.

TM based guns are the ones based on, and usually compatible with, the original TM version of said gun.
MPEG v AEG
AEG, depending on who you ask, either means any TM based electric, or any gun made by the higher end electric gun manufacturers such as TM, Classic army, KWA, or G&G.
MPEG is generally generally defined as lower priced (100-ish dollar range) versions of the higher priced guns. They are often not that much worse in performance, however, only durability. Some companies that make MPEGS are CYMA, JG, Echo 1, SRC, D-Boys, and UTG. I can personally vouch for CYMA and JG.

Step 2: How AEGs Work

For the purposed of this article, I will devide the AEG into three systems:
1: Hopup and Barrel
2: Gearbox
3: Electrical system

There will probably be information that I miss simply because there is so much information out there.

Step 3: How AEGs Work: Gearbox

The gearbox is perhaps the biggest step in understanding the functionality of AEGs.
Simply put, a switch activates a motor, which drives three gears ( bevel gear, spur gear, sector gear in order from motor). The last of these gears pulls a piston back, then releases it when it come to the end of its teeth. The piston is then driven forward by a spring, which pushes all the air out of the cylinder and through the barrel.

The gear on the motor is called the pinion gear. The anti-reversal latch prevents the gears from running backwards.

All TM based gearboxes work like this, with some minor variations in compatibility and placement of parts, along with gear box shape.

This list show which gear boxes are in which model of gun. This may not be 100% accurate for all TM based guns.
Version 1:
FA-MAS F1, FA-MAS SV
Version 2 :
CAR15, M16A1, M16VN, M16A2, M4A1, M4A1 RIS, SR-16, XM177e2, M733, M4-S, G3A3, G3A4, G3 SG-1, MC-51, G3 SAS, MP5A4, MP5A5, MP5K, MP5K-PDW, MP5SD5, MP5SD6, MP5RAS, MP5-J
Version 3 : G36C, SIG 550, SIG 551, SIG 552, AK47, AK47S, AK Beta Spetsnaz, AUG, AUG Military, UMP
Version 4:
PSG-1
Version 5 :
UZI
Version 6:
P90,Thompson
Version7:
M14

Animation is by Cheeshead of AIrsoft retreat. subject is a version 2 gearbox.

Step 4: How AEGs Work: Hopup and Barrel.

The hopup is the system that puts backspin on the BBs to make them go farther and be more stable in flight. It consists of a few basic parts:
- bucking (or rubber)
This is a rubber or silicon sleve that goes over the breach (or back) end of the barrel and extends into the barrel through the barrel window and makes contact with the BB. The friction of this contact is what gives the BB backspin.
-hopup arm
The adjustable peice that puts preassure on the bucking via the nub
-nub (or spacer)
This is a small cylinder that fits between the hopup arm and the bucking.
-Hopup chamber (often just called 'the hopup')
The container that all the previously mentioned parts are housed in.

Step 5: How AEGs Work: Electronic System.

The stock electronic system in AEGs is quite simple: a swich activates a motor powered by a battery.
The battery is a collection of 'cells' each of which are 1.2 volts. This means that:
a 7.6v battery has 6 cells
a 8.4v battery has 7
a 9.6v battery has 8
and a 10.8v battery has 9
The battery is connected to the gun by a connector called the "Tamiya" connector. It is a polarised plug, meaning that it can only be hooked up one way. Airsoft guns use an airsoft specific mini-Tamiya connector that has the opposite polarity of the normal mini-Tamiya connector.
Batteries have something called mAH rating. this stands for "milliamp hours" and is the capacity of your battery. higher mAH means higher capacity.
The motor is a simple electric motor. Not much to say about it.

Step 6: Upgrading AEGs

The 'upgrading AEGs' section will be divided into the following sections:

Range/accuracy
Rate of fire
Stealthiness

These sections mimic common paths for upgrading. And will be followed by other random things that don't fit nicely into any of those three.

There will be no disassembly instructions, as this varies by gun. The information included will be fairly basic.
For more in depth reading, I found a some qualified authors:
For shimming, hi speed, stealthiness and other good info:
http://pageproducer.arczip.com/daedalus03/workshop.html
For long range and some stealth stuff:
http://airsoftsniper.proboards.com/index.cgi?board=aeg&action=display&thread=1291&page=1

I would advise anyone wanting to upgrade an AEG for the first time to first replace the battery with a better one ( I recommend a 9.6v pack with 'Elite' cells), as this is the easiest way to get a very noticeable performance boost. Also, the capabilities of a gun are directly limited by the battery. You can build a gun that shoot 500fps at 25+ rps ( and people have), but you're gonna need a powerful battery.

Some of the most basic things you can do to improve performance are as follows:
Re-shim and re-grease
Clean out your gearbox and put in fresh grease ( you don't need much. Ideally you want a thin uniform film of it). Get some shims ( they're really cheap ) , and starting with one shim under each gear, rearrange the shims till the gears run smoothly without much lateral play. this will require you to open and close the gearbox repeatedly till you get the configuration of the shims right, but it's worth it.
Compression
To improve compression you can:
Get an o-ring air nozzle
wrap your cylinder head with Teflon tape
stretch your piston head o-ring
Wrap your bucking in Teflon tape or "whip" it with floss.
Sand a fraction of a millimeter off the front of your tappet plate.
This will improve the efficiency of your system ( and therefore slightly improve muzzle velocity), and helps to make it more accurate.
MOSFETS
A MOSFET is a solid state switch that lowers the resistance of our electrical system. It can be bought, or made at home. It is especially useful if you are using voltages above 9.6v, as it eliminates electrical arcing between the trigger contacts. I will try to explain them in another instructable in the future.
Other things
You should get a spring guide with bearings and (optionally) a piston head with bearings so that the spring will decompress the same way every time, making your FPS out put more consistent. Also, NEVER, ( well, in rare cases) use plastic bushings. they should be replaced with metal ones.
Switch to Dean's Ultra Plugs. The stock tamiya connectors will break easily and carry lots of resistance. Deans plugs have neither of these problems.

Step 7: Upgrading AEGs : Range and Accuracy

This is one of the more common paths attempted be those new to upgrading. It is also one of the hardest. To get truly sniper rifle like performance out of your AEG, you have to have lots of time, patience, and money. Essentially, your looking to make your gun as stable as possible. That means sealing any air leaks, and shimming or taping where applicable.

That said, lets jump right in.
The most important factors in accuracy, rated from most important to least important, are as follows:
1: BBs
2: hopup bucking / nub
3: Barrel
4: FPS

Note that FPS is last. It is a common misconception that FPS = range. This is simply not the case. Increasing muzzle velocity can actually DECREASE accuracy by causing BBs to 'knuckle' ( like when you hit a ping pong ball really hard).

BBs
Buy from a reputable company like KSC, bioshot, or BBking( there are plenty of other good companies). Biovals .3g and .27g are supposed to be amazing, but I can't personally speak to that. Don't worry that certain BB weights will be to heavy for your FPS: .28g BBs have been proven effective under 300 FPS. The only time .2g BBs should be used is for ranges under about 75 feet.

Bucking and nub
A normal bucking with a flat mound should be used with an H, X, or Fishbone nub. This will provide horizontal stability. Prometheus, Systema, and gaurder make good buckings. Madbull and Echo 1 make h and fishbone nubs respectively.

Shredder's Concave Spacer is like an X, H, or fishbone nub, but it is curved in a way that exactly mimics the shape of a BB. With a normal flat mound bucking, it will give about the same results as an X, H, or fishbone, but if the mound is removed, it will give better, and supposedly stunning, results. to remove the mound, turn the bucking inside out, and sand it off with 400-ish grit sandpaper. Then wash the bucking off and turn it back rightside-out.

The KWA 2GX and Firefly buckings have split mounds that give the same or better results than a normal bucking with an X, H, or fishbone nub. They must be used with a normal cylindrical nub to be effective.

Without buying anything, you can improve accuracy buy washing your bucking, thereby removing any gunk that came from either the factory or normal use.

Lastly I'd would like to mention this new thing called "flat hop" or "g-hop" that people have been using recently with great effect. The concept behind both is that a longer contact region between the bb and bucking provide superior range. This mod is not for those uncomfortable with putting lots of time and effort into a gun, or those with little technical expertise. It is discussed here: http://www.airsoftforum.com/board/Flat-Hop-t220183.html

EDIT: I have recently tried flat hop with a systema bucking and a nub made from an eraser. The distance at which I got a 14 inch grouping went from 150 feet to 180 feet, and I have gotten BBs out beyond 200 feet. This is at 1.5 j (or just over 400fps with .2s)

Barrel
Don't bother getting a longer barrel: It wont help much, if at all. The dimensions of the barrel (diameter and length) matter almost not at all compared to the quality of the barrel because the tolerances of the barrel are much lower (better) than those of the BBs.Get a barrel from a good company and don't worry about getting a longer, tighter barrel.
Some good barrel manufacturers are (in no particular order):
EdGI
PDI
Prometheus
Nineball
DB Customs
Guarder

FPS
If you want to upgrade FPS (which you should do LAST) you will need a few things:

Stronger spring
Better battery
metal bushings
Metal spring guide with bearing
Metal cylinder head

The reason for all these upgrades is that a stronger spring will put more stress on your other parts, so they need to be upgraded in kind. The bearings on the spring guide and piston head allow the spring to rotate, so that it will always decompress linearly. These, along with an O-ring Air nozzle, should be used anyway.
If you want to go above 400fps you may also need:

Torque gears
Torque motor
Solid metal bushings

Whether you need a reinforced gearbox depends on the version. Version 1 gearboxes have been known to crack as low as 340 fps, ans so should generally not be used for this.
Many modern v.2 guns can take 400fps no problem, maybe with minor modification. The same is true of v.3, though v.3s are usually considered stronger than v.2s.
V4 through 6 I can't speak to, but v7 is a very good gearbox, that will most likely hold above 400 fps.
SAW gearboxes are tanklike: We're talking very high fps at very high rps.

If your gearbox has 6 mm bushings, it may be a good Idea to get a new Gearbox any way so that you can use larger bushings. The larger the bushing, the stronger it is.

Step 8: Upgrading AEGs: Rate of Fire

High rate of fire ( or ROF ) is good for CQB, or support gunning. While the gear box strain is less, a high ROF setup goes through many more cycles much faster, so it will wear out faster. This is exacerbates small problems, so things like gear shimming and motor height must be as perfect as possible. Also, the same things should be done to the hopup and barrel that would be done in a long range setup.
It can be achieved with
A Better Battery
Faster motor
lower power spring
lower gear ratio
less gearbox friction.

Battery
A 9.6v battery will give you a good rate of fire on stock gears with a lighter spring. a 10.8v will give you respectable good ROF and 11.1v LiPO will produce walls of plastic. LiPOs require lots of care though, or they will literally explode.

Motor, spring, and gears
A motor designed for high speed will not be able to pull as strong of a spring, but it will spin much faster. lower ratio gears (under 18:1) will make the sector gear spin faster than it would with normal ratio gears. A lower power spring will be eaisier to pull back, so it will be pulled back faster.
It is important to note, however, that at a high enough rate of fire, the sector gear can engage the piston before it is all the way forward. This is catastrophic.
To prevent this, you can "swiss cheese" the piston to make it lighter and there for fly, or you can take off a few sector gear teeth from the engagement side and slightly increase the spring stiffness. Both these things shorten the time it takes for the piston to get all the way forward.
Dual sector gear: If you want really high ROF, get a RiotSC dual sector gear. Essentially, this gear short strokes the piston by half its teeth so that it can have two sets of teeth, thus firing twice in one rotation ( don't worry, semi still works). This gear has been known to produce ROFs in excess of 50 rps. Note that you will need a fairly strong spring (400fps equivalent or higher ) to keep any reasonable amount of power.

Less Gearbox friction
Using ball bearings instead of bushings will lower the amount of spinning friction for the gears, but will be unable to handle higher power springs.

Sector chip
Adding an extra piece of material to the nub on the sector gear will pull the tappet plate and air nozzle back farther, which should increase feeding consistency. There are some companies that make them(some stock JG gears have them), but most are homebrewed.

Swiss Cheeseing
A lighter piston will be faster to pull back, as it has less mass to accelerate. You can drill holes in the piston (except on the rails and teeth) to lighten it. This is called swiss cheesing, and is generally done only on hi speed setups.

Step 9: Upgrading AEGs: Stealthiness

While an AEG cannot be truly 'silenced' it can be muffled to the point where it is inaudible from well within its effective range. This can be done in a few ways:

Padding
Padding the cylinder head and piston head will quiet the 'slap' that occurs when they hit each other. this can be done cheaply with the foam that mouse pads are made out of, but many swear by 'sorbothane' or 'thermoplastic elastomer'. If this is done, the second tooth of the piston must be removed so that it still properly interacts with the sector gear. This is called 'correcting the angle of engagement' . It is a good Idea to do this any way, as it will reduce stress on both your gearbox and your pistons first tooth.
Also, any space inside the gun that is not occupied by the gearbox can be filled with sound dampening padding.

Gears
Once again, a good shim job will be quieter than a bad one. Also, helical gears are supposed to make less noise than normal ones. I cannot speak to them being quieter, but they are a bit harder to shim.
Shimming gears can be time consuming, but its worth it.

With a fast trigger response, the gears will make noise for less time. This can be achieved with a torque motor and speed gears.

Suppressors
Foam filled suppressors can take dampen the sound some, but their legality could be an issue. If you absolutely must have a functional suppressor, you can make it (more) legal be permanently attaching it to your gun.