What is Joule Creep?

Joule creep is a phenomenon that happens in airsoft that effects how ‘powerful’ your airsoft gun is. It’s a dangerous thing that can leave your airsoft gun being too powerful to use, or even illegal under the latest UK Law (read here for more information).

In a recent article we describe how measuring a gun’s FPS alone is not an accurate depiction of the gun’s FPS when actually used and Joule Creep is simply another aspect of how issues can arise without proper knowledge and correct testing.

When we measure a gun’s muzzle energy, we tend to measure using a 0.2g bb. This is because sites generally have 0.2g BBs that they can use as a verified weight of BB, rather than taking the player at their word. A player with a bolt action sniper rifle fires through the chrono and the muzzle velocity is recorded as 2.3 joules when using 0.2g BB. The site is happy that the rifle is within the site’s FPS limits and the player is allowed to play.

However, snipers don’t often use 0.2g BBs in their rifles; they use heavier weights for greater accuracy and range. So if the player decides to use a heavier BB, like 0.4g. The same bolt action sniper rifle, in certain cases due to the joule creep phenomenon, could now be firing at 2.7 joules of energy (0.4joules higher), despite no changes to spring strength (or HPA pressure etc).

This is joule creep: A physical change in the output energy of a rifle simply due to the weight of the BB.

What causes joule creep?

When a 0.2g BB is fired down the barrel, it is pushed by the compressed volume of air behind it until the BB leaves the barrel and can no longer accelerate.

When a heavy BB is fired, it too is pushed by the compressed volume of air; however it takes longer to leave the barrel (as it’s moving at a slower speed). If there is sufficient air behind the BB, it will continue to accelerate, giving it more energy than lighter weight BBs.

This is caused by ‘over volume’, where more air than is needed is used to propel the light BB, when the heavier BB is fired it has more time to utilise this excess air pressure, resulting in higher muzzle velocities.

Counter intuitively, this is most prevalent in short barrelled rifles. Mainly due to the fact that long barrelled rifles require a lot more air to ‘over volume’ them, making it much harder to do. 

The end result is a gun that fires well within the limits using 0.2g BB, but when the player uses higher weights, they are shooting at a higher muzzle velocity. This is often accidental, but some players are beginning to build their rifles to take advantage of this fact.

The Dangers Of Joule Creep

The latest updates to the Policing & Crime Act puts a very specific limit on airsoft weapons, anything found to be over this limit is no longer classed as an airsoft weapon (See our post here for more info). There is a very real chance that airsoft weapons, be it accidental or deliberate, could be caused to be over the limits simply by changing the weight of ammunition being fired.

We suggest that you chronograph your airsoft gun’s FPS with any and all of the weights of BB that you intend to use in the weapon and calculate the joules that your weapon fires at. This is the only way of ensuring that you are within the limits.