Bob’s Guide To Tuning Unregulated PCPs

Today, HAM’s Technical Editor, Bob Sterne gives the benefit of his experience in tuning unregulated PCPs.

In previous articles in this series I talked about how air pressure, barrel length, hammer strike and port size affect the power output of a PCP airgun. For the most part, the manufacturer sets those parameters, or you may have chosen to modify them to meet your needs. So, now you have a given setup, capable of a given performance, and you want to optimize that performance by “tuning”.

For this article on tuning unregulated PCPs, we will assume that the fine tuning will be done by varying the hammer strike.

Generally, this is done by changing the hammer spring preload, or by adjusting the gap between the spring and hammer to change the “free flight” and cocking distance, if you are using an SSS or SSG. Both systems have the same net effect. More preload or a smaller gap increases the hammer strike, and vice versa.

Before we get into the details of tuning unregulated PCPs, let me deal with one other thing…. pellet weight.

Generally in PCPs, a heavier pellet, while reducing the velocity, will actually increase the FPE. However, if you go too far, the gun simply cannot keep up with the demands, and the FPE will eventually fall, like this:

Bob’s Guide To Tuning Unregulated PCPs

In that particular gun, once the pellet weight exceeded 80 gr. the FPE began to decrease. The point is that you can’t keep increasing pellet weight forever in the quest for more power, without other (often drastic) modifications to the gun.

The Basics of Tuning Regulated PCPs

Once you have chosen a pellet that meets your needs, the basic parameters of the gun are set. Now you will want to know how to optimize your gun within that.

The first thing you need to know is that there is more than one “right answer” when tuning unregulated PCPs! Oh, great! Another decision to make!

Consider the chart below, noting that for each shot string there is a corresponding colored line showing the pressure drop:

Bob’s Guide To Tuning Unregulated PCPs

First, look at the black string on the upper left. This gun can use up to an 1800 psi fill, and with the hammer spring preload set 2 turns out from maximum, that is what you get.

The first shot is the fastest, subsequent shots are slower, and there are 22 shots available before the velocity falls 4% below the peak, at about 1500 psi.

This was almost universally known as the “Korean Cliff” tune, because these guns tended to be tuned to produce the maximum possible power on the first shot. Unfortunately, since “velocity sells”, this is becoming increasingly common, instead of less so.

We’ll simply call it the “cliff”.

I now bring your attention to the teal colored string, where the preload was 4 turns out. This string starts at 1800 psi, and ends at about 1300, and has 52 shots. Obviously the efficiency is greater than the previous one, over twice as many shots at about 10% less FPE, on a 500 psi drop instead of just 300.

Most times, this will be the result you will be looking for when tuning unregulated PCPs…. a symmetrical bell curve.

As you continue to back out the preload, the string gets longer, but you will notice on the chart it shifts to the right (to lower pressures). This is because if you fill to 1800 psi, the first shots will be more than 4% below the peak velocity.

The purple string, at 5 turns out, starts at about 1700 psi, and ends at about 1100 psi, but has 64 shots. If this tune suited your needs, no reason not to use it, and only fill to 1700 psi.

Therefore, in an unregulated PCP you can have many acceptable tunes. Each will have its own corresponding pressure range. Generally, reducing the hammer spring preload will give more shots, at greater efficiency, but may require a lower fill pressure.

What is Actually Happening?

You will remember from previous articles, I mentioned that for any given pressure, there is a plateau velocity set by the parameters of the gun.

No amount of hammer strike will increase the velocity further.

Conversely, on the other end of the curve, the valve is operating in “partial valve lock”, and not opening fully, because of insufficient hammer strike for the pressure. If you plot the velocity vs the hammer spring preload at various pressures, you get something like this theoretical chart:

Bob’s Guide To Tuning Unregulated PCPs

Increased preload is to the left.

Note that as you increase the pressure, the velocity is higher, and the point at which the plateau starts (the knee) requires more preload. In the middle of this chart, the curves cross over each other.

If we cut vertical “slices” through the chart, at various preloads, we get another chart that looks like this:

Bob’s Guide To Tuning Unregulated PCPs

The red line is your classic cliff.

There is too much preload for the operating pressure range, and as the pressure falls, so does the velocity.

As you reduce the hammer spring preload, a bell curve starts to appear (yellow), and as you reduce it further we get the “best” tune for this pressure range (black). If you reduce the preload further, the valve does not open properly at the highest pressure (teal), and if you want to stay within a narrow velocity range you must use a lower fill pressure (in this case about 2800 psi).

As you decrease the preload even more (purple), you would start at an even lower pressure (eg. 2600 psi) but you would also shoot down to a lower pressure as well (perhaps 1600 psi).

What is an Acceptable Velocity Spread?

No discussion of tuning unregulated PCPs would be complete without a discussion of what is an acceptable spread between the highest and lowest velocity in the shot string, known as the ES, or Extreme Spread.

I almost always use a 4% ES as my standard, meaning the slowest shot is not less than 96% of the fastest. I find this acceptable for shooting within 50 yards.

The shorter the range, the wider the ES you can tolerate before you notice vertical stringing on the target. For plinking at 20 yards, a 10% ES may be acceptable.

If you are shooting at 100 yards, you might desire a 2% ES or less. However, don’t obsess on this number. The very best .22LR target ammunition has an ES of over 2% over a box of 50 shells. Typical Long Rifle High Velocity ammo is more like a 4% ES.

It depends on your pellet choice (a pellet with a good BC will have less vertical stringing at the same velocity), but generally the above recommendations will have any stringing hidden within the nominal group size of the rifle and shooter.

That deals with tuning unregulated PCPs. Next month we will talk about one method to, at least in theory, reduce the ES. Pressure regulation.