Batteries, Motors & Gearing for ROF!

Considerations for motors, gearing and battery combinations.

When building a gearbox for an Automatic Electric Gun (AEG), any deviation from the original parts fitted by the brand manufacturer (such as a change of battery), can have positive or negative effects if not thought through properly.

Whatever changes you make, you can be sure the Rate Of Fire (ROF) will be affected for varied reasons.

Why should I be mindful of changing these parts?

If you unknowingly (or knowingly) increase the ROF of your AEG it could cause internal damage very quickly, cause feeding problems or prematurely wear out your gearbox.

Battery voltage change;

The effect of battery voltage can be thought of like the rev limiter in your car or on your motorbike, if you increase battery voltage you increase the Revolutions Per Minute (RPM) the motor is capable of – and therefore the speed of all the internal components of your AEG gearbox.

Likewise if you reduce the voltage the motors maximum RPM will decrease, this is why a drained battery makes your gun fire slower.

Voltage drop;

A good example of an easy gain in ROF within the standard limitations of any gearbox is the fitment of a MosFet and a quality re-wire, this is because of an effect in electrics known as ‘volt drop’.

Volt drop in an electrical system is caused by resistance in the electrical system.

Resistance is caused by restrictions such as poor quality or undersized wiring, corroded or burnt contacts and connections for example.

Resistance consumes a percentage of the battery voltage whilst overcoming these restrictions, meaning your motor ultimately gets less voltage.

This effect can be seen clearly here; Mosfet install on a brand new gun!


Motors come in all sorts of flavours resulting in different RPM and torque outputs.

Motors are commonly described by the Turns Per Armature (TPA), this is the number of windings around the armature.

Higher TPA / more windings = HIGHER TORQUE / LOWER RPM

Lower TPA / less windings = LOWER TORQUE / HIGHER RPM

With this in mind you can see that just like changing your battery voltage, a motor with a different RPM range will also change the speed of all the internal components of your AEG gearbox.

It’s also worth noting here that the type of magnet in a motor can also give similar effects, rare earth magnets like Neodymium are much more efficient and can give similar torque output to higher TPA Ferrite magnet motors without loss of RPM, a handy bonus if you want to pull a stronger spring without losing any ROF!


Gears are quite a substantial set of parts to change and are not usually changed accidentally!

The 3 gears inside an AEG work together as a set to achieve a drive ratio.

A low gear ratio, is written as a higher number, for example 18:1

A high gear ratio, is written as a lower number, for example 10:1

The ratio means that it takes 18 (or 10) revolutions of the first gear to fire the AEG once.

Many gear ratios are available however the most common ratio is 17.28 (18:1) in a standard AEG.

You can calculate your gear ratio however the tables below give a fairly good indication without opening up your gearbox and inspecting the gears.

Looking at the whole picture;

From the tables below, with the use of some basic knowledge you can learn what your current setup is and see how you could change that setup should you want to.

These are good average figures to work from.




3 thoughts on “Batteries, Motors & Gearing for ROF!

  1. Do you know what gear ratio would work the best with a m120 spring?


    1. Hello! There are many other factors that need to be considered to make a suggestion for the best gear ratio, you could run any gear ratio with a spring of that strength/power!


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