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How to adjust FPS in an Airsoft Replica? [Essential Guide]


This article covers everything you need to know to manipulate the power of your replica. Sounds risky? It surely can get risky if you tune your rᎥfle higher than your country’s limits. First of all, you’ll get into trouble, you won’t be allowed to play if “chronoed” before a game and also you might potentially harm others and yourself. For example, some eye protection equipment is only rated just above certain legal Joule limits; also, a huge tip – always wear eye pro when shooting, even when just chronoing your Airsoft replica. 

To clarify, joule is a unit of measurement for kinetic energy held by a projectile. There are two factors that add up to joule energy; mass and velocity of said projectile. This energy is then equal to force of newton meter when the projectile connects with another object. Let’s put it in perspective:

  • AEG (UK) = 1.2j – 350FPS at 0.20g
  • Airsoft Sniper rᎥfle (UK) = 2.3j – 500FPS at 0.20g
  • Over the counter Airg⊔n = 20j
  • 9mm round = 600j
  • 7.62 AK round = 2,100j

Even though you have nowhere near the firing power of an AK, you can still potentially cause harm to other people. Even a 1.3 joule replica can penetrate skin or chip a tooth from a fairly close distance. The limits issued by sites in your area are there to prevent that happening whilst ensuring you have fun playing.

When you use heavier BBs, remember to calculate accordingly as while you can be under the FPS limit, you might be well over the Joule limit thus being dangerous to other players. The reason being, heavier BBs carry more energy due to increased mass; below is a representation of how weight impacts Joule energy at the same FPS velocity.

  • 0.2g   @ 350fps = 1.2j
  • 0.45g @ 350fps = 2.56j

So focus on adapting your replica to Joule limits, rather than FPS limits when using heavier BBs than 0.20g. If your replica is reasonably under the limit when using 0.20g BBs, it will most likely remain under the limit when using heavier BBs. If it’s right on the edge, you might have to be careful in regards to “Joule Creep”. As mentioned previously, heavier BBs carry more energy due to increased mass, thus they might somewhat increase the energy output and push your replica slightly over the limit. 

Now that we have health and safety out of the way, we can focus on how to adjust FPS (and Joules) in your airsoft replica. However, if you are not upgrading the internals along with the spring, you can skip step 2.

Read More: if you’re still unsure about FPS and Joule limits, click here for a dedicated article where we’ll walk you through the exact FPS/Joules your replica should be shooting at.

Changing FPS in AEGs and Spring RᎥfles

Step 1Learn how to disassemble your replica

Kind of obvious, but it is important to take your time and follow instructions precisely when opening up your replica as lack of knowledge and rushing can lead to shooting at 0 fps which nobody is after. If you are interested in fine tuning your replica to be just under the limit and maximising its performance, prepare yourself for numerous disassembles.

Patience is key!

Step 2 – Upgrades first! 

Once you’ve disassembled your replica and you plan to upgrade the internals alongside adjusting the FPS, this is the moment to insert all of your new shiny parts! It’s important to do so before trying to adjust the FPS as changing any internal parts, such as: inner barrel, cylinder, piston, nozzle etc. will affect the FPS of the replica due to better (or worse) airseal. 

If you don’t feel comfortable doing your own upgrades, ask your local tech for help or send it over to our recommended store – Camo Raids – for best quality and service. 

Step 3 – Chrono

It’s really important to find your starting FPS in order to insert an appropriate spring when ready; remember, you are trying to maximise the replica’s performance whilst keeping it under your country’s FPS limit. Measuring by how many beer cans your replica can shoot through, just simply doesn’t cut it in this scenario – you will require a shooting chronograph to get precise measurements, see the highly praised and accurate XCortech X3200 below!

Recommended Chrono

Step 4 – Spring 

Spring and battery (AEG) powered airsoft replicas are the largest cohort on the market and the strength of the spring is the main factor for their FPS. They are of different lengths for different platforms, and have various thickness to fit limits within certain region of the world. We will discuss CO2 and Gas replicas in the next section; for now, there are three ways in which you can manipulate springs to reduce or increase FPS.

First, simplest and the most recommended option is shopping for a different spring. Many leading companies offer a variety of springs which are rated for different FPS. Be aware that FPS suggested by the company does not mean that your replica will perform exactly at that level when you change it. There are many other factors affecting FPS in airsoft replicas meaning you have to chrono it before the changes (See step 3). On top of that, new springs have a “break in” period which subsequently means that FPS will change slightly after being used a couple of times.

Recommended Springs on Ebay

Second option involves irreversible action which can be done with a grinder/sandpaper and a blowtorch/gas oven. Cutting the spring is a great way to slightly manipulate and slightly reduce FPS. If you are just over the FPS limit, you can go ahead and cut it down by one coil. After that, you are going to need to bend the end to create even surface (preferably by heating it up with a blowtorch, or gas oven), just like it was originally. Finally, use sandpaper to make sure the material is as smooth as possible. Uneven surface can cause damage and increased wear and tear inside the replica’s cylinder. 

Third option which is uncommon – compressing/stretching the spring. The spring is made of material which is not invincible to changing shape thus it can be slightly manipulated. If you keep the spring compressed for a long time or you have the tools to leave it overstretched, you will notice that its capability and length has changed.This is particularly useful for Sniper RᎥfles when they’re shooting just over the limit; leave the rᎥfle cocked (thus spring compressed) for 2 or 3 nights before the game to lose up to 15 FPS and stay within legal limits.

Keep in mind that if you change the spring to drastically increase FPS, it might have a huge impact on other components such as gearbox, trigger assembly, piston and cylinder. Most companies will produce a well balanced rᎥfle that shoots at certain FPS adequate to the quality of the internal parts. If the spring you insert is much stronger, this might drastically reduce lifespan of said components.

Step 5 – Chrono Again

The final step to ensuring correct FPS before you actually take your replica out for a game! However, be prepared to disassemble your rᎥfle a few more times; it is unlikely the rᎥfle will shoot exactly how you want it the first time around. Do not worry and load with patience! 

If necessary– repeat steps 4 and 5 until desired results are achieved.

Read More: to further optimise your replica’s performance, just stable FPS is not enough – make sure to learn about upgrading hop-ups to significantly increase range and accuracy in our ‘Ultimate Guide to Airsoft Hop-Ups’.

Changing FPS in GBBs (Gas Blowback Replicas)

This can get somewhat tricky depending on how far are you willing to go for desired result. Fine tuning your Gas rᎥfle can be very difficult due to the inconsistencies in FPS in regards to external conditions and the way they are constructed. It is important to mention that temperature affects the gases’ pressure thus the FPS – the hotter, the higher FPS; the colder, the lower FPS. 

Variety of Gases

The first and the recommended method available is the variety of different green gas types which perform differently. While the variety is there to adapt to different weather use, if tested in the exact same condition, stronger gases will increase your FPS and vice versa. 

The list starts at Duster Gas (144a) as one of the weakest gases – perfect for pᎥstols with plastic slides and hot indoor CQB sites. Whereas the strongest green gas available on the market is Nuprol Black 4.0 gas – do not use in pᎥstols with plastic slides. This gas performs better in very cold weather, however it will result in much higher power in moderate to high temperatures which might put your replica over the legal limit and can literally break plastic slides. See the recommended gases for different temperatures below.

List of Recommended Gases on Amazon:

Tighter Inner Barrels

Second method is quite simple; changing the inner barrel to a more narrow diameter. Simple physics, a tighter diameter means a higher pressure passing through the barrel thus increasing the FPS. Once you get hooked up on upgrading your rᎥfles, you will most likely upgrade the inner barrel to 6.01/6.03mm diameter to increase accuracy; this might mean you might have to use a slightly weaker gas on the day. 

Fine-tuning springs

Third method is very tricky as it involves working with springs. Again? But you said…  

You’re right, gas rᎥfles are not powered by a spring pushing the piston to compress the air like in an AEG. Instead, gas rᎥfles are engaged by a spring powered hammer which hits the valve with certain power to release gas from the magazine. 

The said hammer spring can be made weaker or stronger to create desired effect. If your replica doesn’t fire after assembly, that means you’ve compressed the hammer spring too much and it doesn’t have enough power to open the valve. You have to be very precise to get your rᎥfle shooting exactly how you want; however remember that temperature is still going to affect its performance regardless of your tuning. 

Also, magazine valves work in a similar way and they have tension spring too. This means that you can adjust every magazine you own separately! Provided you don’t give up after endless hours of tuning. This method is more difficult than the hammer one as it involves more tricky disassembly.

Not Recommended

Whilst browsing Airsoft forums, I’ve found a couple of misinformed guides that are untrue or not worth trying out. 

Changing battery for stronger one – Batteries voltage or current does not affect FPS as it only increases the speed at which the motor spins. This only results in faster cycle and rate of fire.

Cutting the barrel or poking holes in it – Please, don’t.

Porting the piston/cylinder/nozzle – There are some aftermarket pistons that already come ported. Ported pistons have holes to reduce the air pushed straight through the nozzle. Also, these pistons might have other features such as: reduction of noise, better consistency or some other feature.

Putting that aside, you should never port your piston or cylinder yourself. This will most likely create moderate to high FPS inconsistencies and could weaken or damage other parts nearby.

Read More: are you curious how a ported cylinder can increase your replica’s fps consistency – see our ultimate guide on the optimal cylinder to barrel ratio!

Alternatively, you can check out our whole range of airsoft knowledge articles by clicking here!

Final Words

This generally covers everything you need to know about adjusting your replica and working your way up to a perfectly consistent and just under the FPS limit. Let us know if these methods have been helpful to you and please do comment if we’re missing something!

If you do not get desired results or your replica fires at inconsistent FPS, please make sure that your inner barrel is clean and you have good airseal. We’ll cover how to produce perfect airseal in another article so please stay tuned!

Written by Wojtek Lawer

Edited by Kamil Turecki

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