What is a pressure curve you ask? In order to fully explain the pressure curve, I’m going to have to explain a ton of other things before answering that, so bear with me.

Pcp air rifles operate by using high pressure air contained in a cylinder.

Inside the rifle is a firing mechanism. The firing mechanism is very simple. There is a hammer, a valve, and the air cylinder. The hammer strikes the valve and the valve releases a controlled burst of air.

Sounds simple.

When I was building my first pcp rifle, the instructions said to fill the rifle to 2000psi and it should shoot around 800fps. The problem was the rifle was shooting about 400fps and barely using air at all. For the life of me I couldn’t figure out the problem. Once the rifle got to about 1200psi, it would finally speed up.

I was using the wrong hammer spring. The stock hammer spring was for CO2 powerlet (800-1200psi). Once I ordered a Benjamin Discovery hammer spring (for use at 2000psi) it started functioning properly. The hammer spring tension *must* balance the fill pressure of the rifle.

So how do you know if you have the correct balance in your rifle? You find the pressure curve by shooting it over the chronograph. Record your numbers.

Created: 05-07-2016 05:35:32 PM
Description: Crosman 2400 cphp 7.9
Notes 1: 2800psi
Notes 2:
Distance to Chrono (FT): 3.00
Ballistic Coefficient: 1.000
Bullet Weight (gr): 7.900
Temp: N/A
BP: N/A
Shots
#     FPS        FT-LBS     PF
23    896        14.09      7.08
22    905        14.37      7.15
21    915        14.69      7.23
20    921        14.88      7.28
19    925        15.01      7.31
18    931        15.21      7.35
17    942        15.57      7.44
16    948        15.77      7.49
15    952        15.90      7.52
14    958        16.10      7.57
13    958        16.10      7.57
12    969        16.47      7.66
11    974        16.64      7.69
10    981        16.88      7.75
9     980        16.85      7.74
8     979        16.82      7.73
7     965        16.34      7.62
6     973        16.61      7.69
5     964        16.30      7.62
4     965        16.34      7.62
3     ERROR 3
2     943        15.60      7.45
1     896        14.09      7.08
Average: 947.27
StdDev: 27.44
Min: 896
Max: 981
Spread: 85
True MV: 947.60
Shots/sec: 0.09
Group Size (IN): 0.00

Now you input the numbers into a graph… like this:
image

Speed on the right, pressure on the bottom.

Rising arc-As you shoot the rifle, it will shoot slower because the pressure of the air overpowers the strike delivered by the hammer. The valve will not open fully and will result in slower speeds. This is commonly known as “valve lock.”
Apex- As air is used, pressure inside the cylinder is reduced allowing the valve to open properly. For several shots, speeds will be consistent. The strike delivered by the hammer is balanced to the pressure inside the air cylinder.
Falling arc- As even more air is used the strike delivered by the hammer greatly overcomes the opposing pressure from the air cylinder. As a result, speeds are slower and more air is used per shot.

And that, is a pressure curve.

The apex of the shot curve will offer the most consistent accuracy.

That is how you use a pressure curve to your advantage.

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