The Difference Between FPS Vs Joule Measurements With Airsoft Guns

February 17, 2015

If you have ever chronoed your gun, you noticed that you get a “Feet per Second” (FPS) reading but also a “Joule” (J) reading, and have probably wondered what those numbers mean and how they compare to each other. These are two different measurements of what the BB is actually doing once it leaves your barrel. The main factor that changes J ratings is the BB weight, but there are other factors that I am not necessarily going to address in this article like barrel length, barrel bore, and your volume ratio. You can check out my other article on volume matching on the AirRattle blog if you are curious about this.

When comparing FPS and J numbers, it is easy to think about it in terms of automobiles: horsepower and torque. This may be an odd analogy, but it does make sense if you think in these terms. Horsepower is how fast you hit the wall; torque is how far you take the wall with you after you hit it. When applying this concept to airsoft guns, FPS is how fast the BB leaves your barrel or how fast it is traveling between point A and point B in your chrono; J is how much energy it has left to strike a target. Have you ever seen guns advertised with 432 FPS ratings on the box but they are getting this reading with a .12g BB? The gun may be shooting faster (FPS wise) with a .12g, but once that BB gets 100 feet down range it is going to have significantly less energy than a .25g BB shot from the same distance. There is also a general rule of thumb for BB weights and FPS. For every .05g you increase (i.e. .20g to a .25g), your FPS will decrease by around 25.

Your gun is always going to have an optimal BB weight, and if you go above this weight, you will see diminishing returns. For example, if you were to shoot a .40g BB out of an MP5K, more than likely the BB will not go anywhere because it is too heavy and there is not enough air volume to push that .40g BB out properly. It may seem like a lot of work to find your optimal BB weight, but it is rather simple. You want to start with a .20g BB to get your FPS and J numbers. Gradually increase the BB weight, until you start to see your J number decrease.

Let’s say, hypothetically, you measure a 1.4J with a .25g BB, but when you jump to a .28g BB, your J will drop down to 1.0J. This means that your gun does not have enough air to propel that heavy .28g BB, so it cannot reach its full energy (J) potential out of your barrel. What does this mean for you? It is all about trial and error; it just takes time to test different BB weights with your gun.

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