Installing a mosfet is done to limit the burning on the trigger contacts. As additional affects it will increase trigger response, and may slightly increase ROF (Speed). But it's at a risk! And may break your gun. Most of the issues are also addressed here.
"N gate Power Mosfet" This is the main part.
Off load resistor (1.5K ohm). To pull the mosfet to zero.
Gate protection resistor (10-20 ohm). To protect the gate.
Heat shrink isolation rubber.
A mosfet in a gun is used as a switch (YES instead of your trigger!) that when turned on allows current to flow to the motor. A Mosfet has 3 pins "Gate", "Drain", and "Source". Gate is the pin that is used to turn on/off the mosfet. When power is applied to the gate, current can flow from drain to source. Drain is where the mosfet takes current from (it drains current) and source is where it flows to. Note that Drain is located 2 places on the Mosfet.
What we do is that we rewire the gun so that the trigger only applies current to the gate of the mosfet (and NOT directly to the motor). This ensures that the gun trigger contacts only have little current on them and therefore they will last longer (endless). Instead it's the Mosfet that takes the switch load.
Current and resistance equals Watts equals heat that breaks your gun. Therefore: Get a mosfet with as low as possible internal resistance (That’s the key to selecting a mosfet!). But remember a mosfet's resistance is NOT static! It depends on gate voltage! And Voltage given from the battery depends on the current draw from the motor!
When you press the trigger of your gun the motor needs to start to spin, this draws ALOT of current. The battery can't supply the full current so the voltage drops. When the voltage drops the Mosfet resistance increase. The increased resistance in the mosfet results in heat. This heat (if big enough) may break the mosfet.
When a mosfet breaks it's essentially just a wire. This will mean that the motor will be connected directly (around the trigger) to the motor = Full auto fire and no way to stop the gun firing! Therefore I ALWAYS recommend installing a safety switch so you can stop the gun before it breaks!
When you buy a mosfet get one that can handle current (100 amps minimum). And that it can handle allot at low voltage (min 20 amps at 4 volts). That it can handle above max battery voltage (+20 volts mosfet). In general download the datasheet for the Mosfet and check the "Main data" and Current curves. Se picture from an IRF4110 It can handle 120 Amp max. And at 4.5 volts around 80 amps. This is Well above the limits of any standard gun, and most upgraded guns!