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What Do Joules Mean in Airsoft?

Airsoft Guns with gear behind

Arguably, one of the most important terms used within airsoft, certainly when it comes to gun safety and operation, is joules. All players should have at least a basic knowledge and understanding of this particular property of a gun, while knowing how they differ and how they correlate. Joules not only has strong implications of how your gun performs when in combat, but also decides whether or not it is safe for use on the field, something that all sites take very seriously and even if your Airsoft gun is legal to own!


In this article, we’ll be looking at how joules affect performance, and how to stay on the right side of the law:

What are Joules?

Man wearing knee pads playing airsoft

A joule is a unit of measurement when measuring energy, typically used when referring to mechanics, electricity or thermal energy. By definition, ‘It is equal to the energy transferred to an object when a force of one newton acts on that object in the direction of the force’s motion through a distance of one metre’.

But why are joules important? The importance of joules comes in when we begin to look at the UK law surrounding the use of airsoft guns. Although many of us, when referring to the power of our weapons, talk in feet per second, Joules are in fact, the final word. When the government made the changes in 2017 to the ‘Policing and Crime Act’, exemptions were made for airsoft guns, good news! This was detailed in section 57A and focused on both fully automatic and semi-automatic guns. Joules were used as the defining factor of power, and the act stated that anything that was ‘capable of discharging two or more missiles successively’ without pressing the trigger again was limited to the energy of 1.3 joules. This, when measured with a 0.2g BB, equates to 375 FPS. For semi-automatic and bolt action guns, the allowance was nearly double, allowing 2.5 joules as the maximum, offering 520 FPS with a 0.2g BB.

Unlike FPS, which merely measures speed, joules actually measures the energy of a BB, taking into account the velocity and the mass. Although joules are supposed to be a constant part of the gun, there are ways that this can change, either deliberately from the user or through joule creep.

What is Joule Creep?

Man in woods playing airsoft

Joule creep is not a myth; it is a genuine part of the game and something that players should be aware of. It’s worth noting that one of the defining factors of joule creep is the amount of air within a system when shooting a BB. Too much or too little, and this can affect the output of the BB. When firing too much air, this can be used to good effect by adding in a slightly heavier BB. Although many AEGs experience minimal joule creep, HPAs and GBBRs can see it happen more commonly.

For example, if your gun is shooting 400 FPS with a 0.2g BB (1.48J), but wasting some air at the end, then you could swap out this for a 0.3g BB, may shoot at 380 FPS (2J) due to the weight but have the BB carry more energy and make use of the wasted air. This results in a higher joule output at the same air pressure setting due to the added weight of the BB. That being said, this is often against site rules, and many often state that when you chrono at the start, you should be doing so with the system setup you intend to use. Excess air from the barrel can also destabilise a BB after it’s left the barrel leading to less accuracy.

It’s also worth noting that it is indeed possible to fire over the limit without actually intending to. Again, unlikely for AEG users, but for HPA and GBBR players, it can be seen. Changes in the temperature can easily affect the output power of the gun, often without the player even realising.

Whether or not it’s intentional, even being in possession of a weapon that fires consistently over the legal limit carries severe criminal weight. If your full auto capable gun exceeds the specified limits, then it could count as a Section 5 firearm, and possession of such carries a 5-year prison sentence. This is why it’s important to have a chronograph in your personal airsoft kit, along adhering to site regulations and keeping your gun firing in line with their chrono readings. Adjusting your gun after having the readings taken is against the rules, and against the spirit of the game.

Keep your airsoft guns firing consistently and legally! Check out our online store here for our range of guns and accessories. You can also find great articles on our blog for tips and tricks for airsoft!

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