AirSharp

Installing a MOSFET System in any AEG
First, some background on MOSFET Fire Control Systems:

When any switch is closed or opened, there's an instant spark at the contacts.
This is because power is trying to flow before the surfaces are completely together,
and when the switch is opened, the current want to resist being interrupted.
That spark is material burning.
After a number of cycles, the condition worsens as the contact points erode and result in increased resistance.
At some point, the resistance is too much, and the switch fails to function.
Now in a stock AEG, this usually won't become a problem during the service life of the assembly.
The contacts in most of them are pretty robust. If your gun is staying stock, you're done reading here, skip this whole process...
But add a stronger spring, a beefed motor, etc, and the contacts have to work harder.
Plan on eventual switch failure, especially if more battery cells were added to the setup.
Modern electrical/electronic systems only use mechanical contacts for VERY heavy loads, that are not frequently switched.
For anything less, the transistor is the solution.
A transistor (in our case, the Metal Oxide Silicon Field Effect Transistor) is a neat little device that acts as an electronic "switch".
Usually, they are used to interrupt the NEGATIVE power lead in a system.
This is the opposite of nearly every circuit that uses switch contacts.
Including AEG's. This is important, because it means the wiring the gun came with needs to be changed in operation.
Instead of the trigger giving POSITIVE to the motor to activate it, we are going to feed NEGATIVE to the motor to turn it on.
I've constructed a simple circuit to illustrate MOSFET wiring:

Note that the negative lead goes into the 'FET and back out, then going to the black negative lead from the gun.
The red, positive lead from the gun goes right to the positive lead from the battery.
The yellow lead is our new trigger wire. When the trigger is pulled, the MOSFET is on, feeding negative to the gun.
Going to the inset pic, the leads on (most) 'FET's are laid out:
GAIN-DRAIN-SOURCE
"GAIN" is the lead that waits for you to pull the trigger or close a switch, giving it POSITIVE voltage.
"DRAIN" is the item needing power, and suitably, "SOURCE" is the power available from the negative side of the power source.

Now you can decide if you even NEED this for any particular gun.

Modify a ver 2 gearbox (M4 style!) for MOSFET or 3-TAP installation:
The stock wiring:

See the small grey part at the bottom? If your M4 trigger feels "notchy", it's in upside-down.
The stock wire is 18gauge, silicone-jacketed. This is suitable for any stock gun.
I'm unsure of how far this will go, so I pulled all the stock wire in favor of brand-new 16gauge.
My new wire is vinyl-jacketed, so the diameter difference is minimal,
which is important for getting everything back together. There is limited space in the gun body for bigger wire.
Silicone is better insulation, but I'll make the sacrifice to use larger conductors.

First, the stock neg lead shown in the top of this next photo:

I'm going to eliminate the part circled, because it does a poor job of keeping the negative lead against the gearbox wall as it goes around the motor shaft.
I learned this little trick from a cheap clone, bend the wire as shown, and it will stay there until you want it back out, if ever.
Note that the negative lead doesn't connect to anything inside the gearbox. It just passes through to get to the motor. Limited room again.

Now to put in the new positive lead. You'll be happier replacing the whole lead with a setup like I show here.
Trying to solder TWO wires to ONE contact will end in frustration, melted parts, or both, unless you're a seasoned solderer.

So I bend a red wire as shown, THEN cut the insulation away from the bend. Now we have ONE wire to solder to the delicate contact,
and we know every conductor is whole from the battery to the motor.
Technically, a whole solder joint and it's resistance are eliminated as well.

Then another red wire is soldered to the other trigger contact. This wire can be really, really, tiny, and the circuit will still operate, as
only a microscopic current is present while firing. In the interest of space/reliability, I went with 22gauge.

Then we button it all up. Next, building a basic MOSFET circuit.

The basic MOSFET install can then be added.