The Beginner’s Guide to Tuning a Regulated PCP Airgun
By Nicolas Gregoris
The airgun market has seen an influx of feature-packed and highly adjustable platforms in recent years. Features like externally adjustable hammer spring tension, regulator pressure, and transfer ports are now commonly available on entry-level and high-end airguns alike.
This wide array of adjustment options is a double edged sword. Shooters with virtually endless options to fine-tune their equipment for any desired combination of power and efficiency but the myriad of adjustment combinations available can intimidate and possibly even confuse new airgunners.
In this blog post we will walk through each step of my go-to tuning method so that you will come away with not only the confidence to tune your regulated PCP but also a working knowledge of the mechanisms involved. The principals and methods we will discuss may be applied to virtually any airgun.
How Do Pre-charged Pneumatic Airguns Work?
Before we can have any meaningful discussion about tuning, we need to have a general knowledge of how Pre-charged Pneumatics (PCP’s) work.
When the trigger is pulled, the hammer spring sends the hammer (or “striker” for our UK friends) barreling towards the valve. Upon impact, the valve opens and allows a burst of air to travel out of the reservoir – the tube or bottle – and down the barrel. The pressure behind the valve wants to force it closed the entire time it is open. The length of time the valve remains open is referred to as “dwell”. When the hammer is pushing on the valve with less force than the valve is pushing back on the hammer, the valve closes and the firing cycle is complete.
“Tuning” is the process of balancing all of these opposing forces. A bad tune really sucks and will take away from your shooting experience. For example, if we have too much hammer strike our valve will remain open after the pellet has already left the barrel. Since that air is no longer accelerating the pellet, it is wasted which reduces our shot count, makes the gun louder and is detrimental to accuracy.
With an unregulated PCP airgun, as air is consumed during the firing cycle, the pressure in the reservoir changes. This changes the velocity in a number of ways. 1) As the air pressure drops, the force behind the air pushing the pellet declines and 2) the lower pressure in the reservoir means that there is less force resisting the hammer spring and there is more “dwell” time. This is generally why the shot curve of an unregulated PCP is described as a bell curve.
On the left side of the peak the pressure behind the valve is too great for the hammer to overcome so the valve is operating in a state of partial “valve-lock” which is a fancy way of saying the valve does not open completely. This creates what are affectionately known as “alligator teeth” – relatively wide swings in velocity – since small variations in hammer strike (caused by friction and other things) effect how much the valve opens and how much air is released.
As air is consumed, the pressure holding the valve closed drops and the increased dwell means that more air is used to push the pellet forward. This is why velocity increases despite the pressure decreasing. The decline in reservoir pressure and volume is linear but the change in velocity is not.
At peak velocity, the valve is opening completely and the pressure in the reservoir is high enough to maintain that relatively high velocity. However, as reservoir pressure continues to drop, the force that the air exerts on the pellet declines so our velocity decreases. You’ll notice that there is quite a large velocity range between our first and last shot and that the most consistent part of the curve is the peak.
The peak is the point where our hammer spring and reservoir pressure are balanced. The problem is that this pressure range only lasts for a few shots. This is where a regulator comes into play.
The chief function of a regulator is to ensure that the pressure available to the valve is the same from shot to shot. The regulator has a set point and does not allow the pressure in the plenum – the area between the regulator and the valve – to exceed that set point. This eliminates the bell-curve shape of the shot string and allows for a more consistent velocity across the entire range of reservoir pressure. In other words, more usable shots per fill.
This is a shot string from a regulated Air Arms S510 – notice the absence of velocity changes and how remarkably consistent the rifle is across the entire shot string. The regulator is set at 150 BAR and the reservoir falls below that pressure approximately 50 shots in, which is why the velocity begins to drop since below the set point the rifle is basically unregulated.
You’ll notice that the regulated part of the shot string for the S510 has a velocity spread of roughly 6 FPS while the unregulated S400 has a spread that is roughly 4-6 times larger so the benefits of a properly setup regulator are obvious. Unfortunately, not all regulated airguns come with a factory setup as beautiful as that Air Arms S510 so it may be necessary to tune your rifle to achieve the desired power, shot count, and/or consistency.
Tuning: How the Pros Do It
For 95% of airgunners we can achieve our optimal tune using only hammer spring tension and regulator adjustments. For the real geeks out there, we can also tune using hammer mass, changing the hammer spring itself, valve spring rate/preload, transfer port size and other things but changing these additional factors makes the process unnecessarily complicated. The 5-Step process below will use solely hammer spring and regulator adjustment. This is a great starting place for those new to tuning.
If your rifle does have valve adjustments (like the FX Impact or Benjamin Marauder). Open them all the way and leave them alone until you have read all of my blog posts 🙂
You will need to consult your owners manual to determine where these adjustments are located on your particular rifle and how to safely perform them. There is a risk you can damage your regulator is you perform the adjustments incorrectly so PLEASE read your owner’s manual. You will also need a chronograph.
Step 1: Begin with a Goal in Mind
The first step in a tune is to decide what performance you want out of the rifle. Do you to produce more power than the stock tune? Higher shot count? More consistency? Pick the projectile you want to shoot and the energy output you desire. Many new shooters will just look for instructions on how to “tune” their airgun without having an idea of what they are tuning the gun to do. It is impossible to make the rifle do what you want if you don’t define what that is.
Step 2: The 95% Rule
In Step 1 we defined our ideal tune. Step 2 will tell us how close to that we are.
At any given pressure level there is a maximum velocity that physically cannot be exceeded. This is called the “plateau”. Generally, the velocity that provides the best trade off between power and efficiency with a regulated airgun exists about 95% below the plateau, hence the “95% Rule”. The specifics of this phenomenon aren’t that important to the tuning process but it mostly has to do with getting the valve dwell dialed in to a point where the valve closes just before the pellet leaves the barrel. That way, most of the barrel is being used for acceleration and no air is wasted.
Since our sweet spot is ~95% below our plateau we want our target velocity to be 5% lower than our maximum velocity at our regulator set point. Our formula for this is:
Target velocity / .95 = Plateau Velocity
For example, if we choose 880 FPS as our target velocity, then our plateau should be:
880 FPS / .95 = 926 FPS
So we are aiming for a regulator pressure that will yield a maximum velocity of 926 FPS. This doesn’t need to be exact – a plateau velocity of 935, or 915 would be a-ok. We are really only shooting for ballpark figures here since tuning to 93% or 97% below the plateau can also work very well.
Step 3: Finding the Current Plateau
Start with your hammer spring tension at a minimum and increase it incrementally until it is maxed out. Record the velocity at each adjustment. If your rifle doesn’t have hammer spring adjustment you can install a beefier spring. You will likely notice that as you increase the hammer spring tension you will reach a point where the velocity stops increasing or begins to increase much more slowly. This velocity is, or is close to your plateau.
The next step depends on where this current plateau is in relation to our target plateau. If our target is a plateau of 926 FPS, but the velocity with maxed out hammer spring tension is too high than the regulator pressure is too high. If it is too low than the reg pressure is too low.
Step 4: Adjusting the Regulator
A regulator pressure that is too high is easier to deal with. When the reservoir pressure falls below the regulator pressure the gun is basically unregulated and the velocity will change with each shot. Leaving the hammer spring maxed out, shoot the rifle until the velocity falls to your desired plateau. Once you reach that point, check the pressure gauge on the gun. This is your new regulator set point. Adjust and move to Step 5.
If you determined your regulator pressure to be too low, you will need to increase the regulator pressure. How much of an increase you ask? It’s difficult to say…you will need to experiment. Leave your hammer spring tension maxed out and increase your regulator set point until you zero in on the desired plateau.
Step 5: Back off the Hammer Spring
The final step is to reduce the hammer spring tension so our velocity falls back down to our target velocity – not the target plateau but the target velocity which in our example was 880 FPS. If you installed a different hammer spring to find the plateau, install a weaker spring (you may need to try several different options).
Once that is done…congratulations! You have now tuned your regulated PCP rifle. It is a good idea to fire a complete shot string just to see what sort of consistency and shot count your new tune will yield.
Generally, an extreme spread – the difference between the highest and lowest velocity – of around 2-4% (16-32 FPS in this case) is considered acceptable for a regulated PCP.
Bonus Step; Enjoy What You Have…Or Not
Airgunners are finicky creatures who are rarely content. If you still have the desire to tinker with your tune we can change the regulator pressure just a smidge. A “smidge” is a technical term meaning 2-3 BAR. Whether you raise or lower the reg by a smidge is up to you, as each option has its place. You will find pro shooters and tuners who advocate both options.
- Increasing the regulator pressure ever so slightly means that the pressure behind the valve is slightly higher and therefore it closes faster. Shutting the valve faster produces a “snappier” shot cycle and generally saves a little bit of air, so it may add a shot or two to the tune.
- Decreasing the reg pressure just a smidge is designed to combat “regulator creep”. Creep refers to an increase in regulator pressure over a period of time. This time period could be as short as 10 minutes and as long as 24 hours. Virtually all regulators have some amount of creep and it is generally very small but this unwanted increase in pressure can still cause the first shot to be of a slower velocity since the hammer has to overcome more pressure. This slower first shot may translate to a lower point of impact and a missed target. Therefore, if we decrease our final reg pressure by 2-3 BAR, then even if we have some regulator creep our velocity will still be the same.
Reg creep occurs after the rifle has been sitting for a while, so Option 1 is generally more popular with competition shooters who will have the opportunity to fire a purge shot before their relay. This purge shot clears the reg and allows it to reset back to the set point. Option 2 is more popular with hunters who may be unable to fire a purge shot, or shooters more likely to need a perfect first shot.
Just because you have a consistent tune does not guarantee that your projectile of choice will be accurate at your chosen speed. There are some projectiles that have reliable sweet spots that seem to work across platforms, but you may need to repeat the tuning process in order to find the ideal speed for your projectile in your platform.
Although this guide will be sufficient for most airgunners, I have alluded to the fact that there are more sophisticated tuning processes and methods available. Those will be investigated and described in the future so make sure you bookmark this blog and check back regularly!
P.S.: If you enjoyed this blog post, check out my video on “Barrel Polishing” which is another way you can make sure that you’re getting the most out of your airgun: