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Updated: Mar 28

Knowledge : FPS Vs Joules & Joule Creep

Many determine the “power” of an airsoft gun by the speed at which it can fire a BB. The most common term for measuring this speed is FPS (Feet Per Second) however this can be misunderstood when it comes to the complexities of different weights of BB and the legal limits as to what’s allowed.

  This information is also available as a downloadable file in our intel section - or you can download from this link HERE


UK Law

There are a number of UK laws in place that both restrict and protect airsoft and it is important to be aware of these. The three main laws are as follows:


· The Firearms Act

· The Policing & Crime Act

· The Violent Crime Reduction Act


In simple - the law defines, a firearm means a lethal barrelled weapon, and a lethal barrelled weapon is defined as a barrelled weapon of any description from which a shot, bullet or other missile, with kinetic energy of more than one joule at the muzzle of the weapon, can be discharged.


However,  the law also acknowledges airsoft guns and the Policing & Crime (section 125) provided an exception for airsoft guns. This exception defines by law what is categorised as an airsoft gun.


As per the extract, there are different acceptable legal kinetic energies for automatic BB guns and semi-automatic / bolt action. However, it should be made clear that you should also defer to the site’s requirements and limitations.

The Maths

Feet per second (FPS) is an easily understood unit of measurement for speed, ie the number of feet that the object will travel in one second. However, as we have already discussed that joules are the legal unit of measurement, we need to be able to determine the acceptable FPS based on the BB weight.


The conversion between Kinetic Energy (Joules) and FPS is derived from relatively simple maths:

To save time and avoid airsofters having to carry a pocket calculator, we have provided a break-down of common BB weights and to the corresponding FPS of the joule output.

This is at the bottom of this article (or in the PDF download).


Joule Creep

Joule creep is a phenomenon where the muzzle energy of an airsoft gun unexpectedly rises as heavier ammunition is utilized, contrary to normal expectations.


When fired, an airsoft gun applies compressed air behind the BB which then forces it out of the barrel. A lighter BB (say 0.2g) will leave the barrel with a certain energy which can be measured by the speed as discussed above.


However a heavier BB will sit in the barrel longer, absorbing more of the force exerted by the air behind it. Other elements can also affect this including different diameters of BB, the use of tight bore barrels and the specific air volumes the firing system is producing. Air volumes can be vary depending on the platform you are using (AEG, GBB, HPA, Bolt Action, etc) with GBB and HPA systems often suffering from “over volume” where the volume of air entering the barrel is set higher than what is needed to launch the BB out of the barrel.


This means that whilst an airsoft gun is tested on a 0.2g BB and found to be compliant with the required muzzle energy (and velocity), the player immediately then switching to a heavier weight of BB for their game day will likely mean an increase in the muzzle energy and could exceed the acceptable and legal limits.


Therefore to ensure safety and compliance with the law, all airsoft guns should be measured using the specific weight of BB being used and deemed acceptable by their joule output.

Conversion Chart

This is available as a downloadable file in our intel section - or you can download from this link HERE (NoFo-002 - FPS & Joules Guide - 1.0)



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