A few questions, please:

1. Every shot will decrease the pressure reservoir pressure slightly. Is the some sort of regulator to provide a constant pressure for each shot? If not have you noticed any change in down range ballistics from first to 40th shot?

There are many YouTube videos that answer this, but, as Ogberi stated, most PCP's don't use a regulator.

Basically, two factors affect muzzle energy and velocity. One is the pellet mass (i.e. weight in grains). The other is speed.

Speed is determined by the differential pressure acting across the pellet as it moves down the barrel (i.e. acceleration).

On a PCP, the hammer is basically a weight that is accelerated by a spring that is compressed when you cock it.

When fired, this gives the hammer some amount of kinetic energy as the spring accelerates it towards the air reservoir valve.

Based on reservoir pressure, this determines the time and amount the valve is open and, as a result, the amount and pressure of the air released.

So at full pressure, the valve is only slightly opened for a short time. As the reservoir pressure drops, the valve is opened more and for longer releasing a larger amount of air at a lower pressure.

This pressure (actually differential pressure) behind the pellet starts to accelerate it down the barrel. As the differential pressure decreases so does the acceleration.

So here two factors affect total velocity based on acceleration, the amount of differential pressure as it moves down the barrel and time the differential pressure acts on the pellet.

Time is mostly a function of barrel length, the longer the barrel the more time the pressure can act upon the projectile.

The pressure, or more correctly differential pressure, is a function of expansion (volume) and barrel length.

As an example, at full pressure lets say the valve releases 1 cc of air at 3000 psi. As the pellet moves down the barrel this volume increases and the pressure resultantly decreases.

For this example let say the pellet moves the first inch down the barrel increasing the volume behind the pellet to 2 cc's. Based on Boyle's law (I believe), the pressure behind the pellet is now only 1500 psi and at 4cc's the pressure is now 750 psi. So as the pellet moves down the barrel, the differential pressure drops as does the acceleration.

Know lets say this dropped the reservoir pressure to 2800 psi. Since the valve will have approximately the same kinetic energy when fired, the valve will be open further and longer releasing more air at a lower pressure.

For the example, let's say it releases 2cc's of air at 2800 psi. The pellet must move twice as far down the barrel to get the same relative pressure drop, i.e half the original pressure.

So for the example, let's say it's 2 inches (1 cc per inch) until the pressure drops to 1400 psi and 4 more inches before it drops to 700 psi.

So in the first scenario, the pellet moves 3 inches when the pressure drops to 750 psi, but in the second the pellet has to move 6 inches before the pressure drops to 700 psi.

Even with a higher initial pressure, the pellet acceleration drops more rapidly in the first scenario due to the faster drop in differential pressure from the ever increasing volume behind the projectile as it moves down the barrel until it reaches the end of the barrel.

That's why if you look at a graph of pressure vs velocity, the muzzle velocity of a PCP air gun for any given pellet weight tends to increase slightly as the reservoir pressure drops to a point where the reservoir pressure lowers to where velocity starts to drop again.