Joule Creep & Testing

I first head of “Joule Creep” after I moved to New Zealand and I was looking to get back into airsoft (I’d had about 6 years away from the game). One of my local sites had an ammo weight limit of .25 for AEG’s and a maximum of .3 for sniper rifles.

I was immediately confused why a site would impose ammo weight restrictions? especially considering back in the UK we would use .25 – .28 on an AEG to help cut through our summer bush (and our FPS limits are generally lower there) the main ammo type I ran in the UK were .36 in my VSR. Occasionally I had tried out Maruzen SGM .29 ammo after being told how amazing they were. But honestly, I really got better results with .36. Then in my AEG’s I would be running Guarder .28’s.

Of course, the first thing I did was ask “why?”. Why is there a limit to what weight ammo you can use?

The answer was Joule Creep.

Now I’ll be honest, back when I first played airsoft, we had never heard of joule creep. Genuinely wasn’t a thing. Well in the sense of it being a ‘thing’, and when I stopped in 2010 it still had never been mentioned at any site or by any players that I can remember.

All chrono checks were done with a .2 BB, and after chronoing you could use whatever ammo weight you wanted.

So what is Joule Creep?

From my research and reading;

Joule creep is essentially the situation that can arise when using heavier ammo in your airsoft gun. This happens when your gun can output a high volume of air behind the BB, and it’s usually gas and HPA guns that are more prone to experience joule creep.

What basically happens is the heavier ammo remains in the barrel for a longer. This in turns allows more air to push the BB out of the barrel giving the BB more initial force and a higher initial joule rating. Lighter ammo exits the barrel before all the air can be pushed down the barrel behind the BB.

This supposedly affects shorter barrel guns more than longer barrel guns (due to the time it takes for the BB to exit the barrel, in a longer barrel gun there is a higher chance that the lighter BB will get the maximum or close to it amount of air from the cylinder behind it. In a shorter barrel, the lighter ammo exits too quickly.

I guess this has been highlighted since gas guns became more popular and HPA became a thing (we didn’t have HPA when I played, you could get conversion kits for bolt action / gas rifles that tapped it for CO2 use, but they were akward and certainly wasn’t as easy as it is now).

Once explained it’s relatively simple to understand. That tiny amount of extra time a heavier BB is in the barrel along with the high volume of air behind it gives it a small amount of extra force behind the BB.

If you’ve got 20 minutes to spare to watch a detailed breakdown from an English dude with a cold, then watch this video.

My Testing

After understanding the theory behind what Joule Creep actually is I decided to do some testing using 2 common airsoft guns.

One gun is a spring powered VSR-10 based rifle. The second is an electric Tokyo Marui NGRS CQB-R.

Specs and results for both rifles:


  • AA Zero Trigger & Piston
  • PSS10 170SP Spring (cut down)
  • Action Army Hop Unit
  • Deep Fire 6.02mm 430mm Inner Barrel


  • Eagle6 M110 Spring
  • ZCI 6.02mm 363mm Inner Barrel

VSR-10 Results

I initially tested the VSR-10 FPS after installing a series of upgrades. This was prior to trimming the spring. With the new spring installed there was a lot of tension inside the cylinder and the piston would stick out the end before the cylinder head was screwed on. After trimming the spring the piston head was level with the cylinder head threads, meaning there was less tension within the cylinder.

I’m noting this due to the only plausible reason to why at first I experienced joule creep, and then eliminating joule creep after the spring was trimmed (as this was the only change).

We can see that first with the uncut 170SP spring we gain the equivalent of 20fps with .2 when using a .3. I’m actually going to get another new spring so I can test with several weights again, including a .36 and see what gain we get with that.

As is stands now, clearly trimming the spring has reduced the speed in which the volume is outputted from the cylinder – enough to pretty much eliminate joule creep. The difference between a .2 and .36 energy output is now 4fps which is minimal and a standard deviation when chronoing.

TM M4 CQB-R NGRS Results

This one is interesting. You can clearly see that using heavier ammo does not result in joule creep. If anything this results in joule loss. This is down to the cylinder not having a high enough volume of air to output against the barrel length / BB weight. Using heavier ammo the cylinder has released its volume of air down the barrel before the BB has exited, and it’s already started to slow down. This is why the joules drop as the ammo gets heavier and the equivalent .2 FPS drops.

Now obviously like I previously mentioned, Gas and HPA powered guns don’t really suffer from this issue as they have the ability to produce a high volume of air to push the BB out of the barrel. This is why they are more susceptible to joule creep. Or if my AEG was properly tuned, chances are the cylinder is ported due to the originally shorter inner barrel as stock which helps prevent joule creep. (See the video mentioned above if unsure what I’m referring to).

I also want to note right now, as HPA guns seem to be a ‘sensitive’ subject after reading around online. I have no problems with them or with players using them. In fact, I’m even looking at getting an HPA system for the VSR-10 because it makes cycling the bolt super light, and the power output is more consistent than spring. Once Wolverine’s new BOLT and WRAITH products have been out for a while and tested / reviewed this is the setup I’ll be looking at.


So, from my testing of 2 common airsoft rifles (bolt action and an AEG), we can see that setting a max ammo weight is a bit pointless due to the fact that it doesn’t always end up in a higher joule / energy output. Also setting an FPS limit using .2 is also a little bit of a grey area due to the fact that joule creep does exist (as proved by the VSR pre-cut spring) and you could, in theory, have an airsoft rifle shooting at a higher FPS (equivalent  joule to .2 conversion) when using heavier ammo (again this depends on the type of gun as it doesn’t affect every gun).

So what would be the best way to Chrono guns, if using .2 isn’t a reliable method, nor is capping ammo weights?

Chrono with the ammo weight the player is intending to use for gaming, and make sure the joule rating is correct.

For example, if your field has a 350fps limit with .2 ammo this is an energy of 1.14j. If the player wants to use .3 ammo in their gun, you simply set your Chrono to read the ammo weight as .3 Chrono as normal and make sure that the joule reading is less than 1.14j (for reference this is about 285fps with .3)

If your site has a 500fps with .2 limit for sniper rifles (2.32j) then you simply enter their ammo weight and Chrono and make sure that their guns don’t shoot higher than 2.32j (fps equivalent for .3 is 407fps roughly, for .36 it’s 374 fps).

At the bottom of this article is an image from that shows a good chart for BB weights, their FPS and joule ratings. You can also use the calculator here that allows you to enter a BB weight, and enter a joule rating to get the FPS.

Most decent Chrono’s have the ability to read in joules and set the ammo weight. The XCORTECH X3200 is a decently priced Chrono and does everything you need to do.

Once I get another new uncut spring for the VSR-10 I’ll do some further testing. I’ll also test a gas blow back pistol and see what results that brings. Once done I’ll update this article.

Your Thoughts

So, what are your experiences with joule creep? Have you done any testing? What were your results? I’d be keen to hear. Have I got something wrong? Let me know in the comments below!


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