Learn › Track 02 · Equipment & physics
Air pressure and the shot
Over a long training session your cylinder might fall from 200 bar to 100 bar. That is half the pressure behind the valve. Do your last shots leave the barrel as fast as your first?
Think first: Imagine a PCP pistol with no regulator. The cylinder goes from 200 bar to 100 bar over a session. Do the first and last shots leave at the same speed?
- Yes — the valve opens the same way every time
- No — the push on the pellet depends on the pressure behind the valve
- Only if the room temperature stays the same
The regulator
A regulator sits between the cylinder and the firing valve. It lets air through until a small chamber reaches its set pressure, then closes. Every shot gets the same charge, whatever the cylinder reads, as long as the cylinder stays above that set pressure. In one popular model the regulator holds steady from 200 bar down to about bar.
Drag the slider to use up the air.
While the cylinder is above the set pressure, the regulated velocity is a flat line. Once the cylinder falls below it, the regulator can’t fill the chamber any more. Every shot is a little slower than the last, and your shots start to go low.
Speed and height
Why does a slower pellet land low? Because gravity has longer to pull it down.
Think first: At 150 m/s a pellet takes about 0.07 s to cover 10 m. How far does gravity pull it down in that time?
Drop goes with 1/v², so small speed changes matter. Each 1 m/s of difference between shots moves the impact by about 0.3 mm. A spread of 5 m/s between your fastest and slowest shots stretches the group by about 1.5 mm up and down. That is nearly two decimal bands, before you have made a single mistake.
So when a group is strung vertically, the air can be one cause. It is easy to test: a chronograph shows your extreme spread directly. Breathing, grip and light changes can string shots vertically too. See Reading your target.
Shots per fill
How many regulated shots you get depends on the cylinder size and the pistol. Makers give these figures for a 200-bar fill:
| Pistol | Shots per fill (maker or review) |
| Feinwerkbau P8X | about 160 |
| Walther LP500 | about 150 |
| Morini CM 162EI | 150 |
Learn what your gauge reads when you are close to the end. For one pistol, the manual says about 30 shots remain at 100 bar. A qualification match is 60 shots plus as many sighters as you choose, and a final adds 24 more.
Think first: You have made the final. Your gauge reads 110 bar, the regulator is set at about bar and each shot uses about bar. What do you do?
- Nothing — about 40 shots left is plenty for 24
- Top up, or swap to a full cylinder, before the final
- Shoot the final and top up if shots start going low
Temperature and altitude
A regulated PCP is far less sensitive to temperature than CO2, but the cylinder itself still follows the gas laws. Its pressure changes by about 0.34 % for each °C. That matters when you fill, because a warm cylinder reads high. It matters much less for the shot, as long as the cylinder stays above the regulator’s set pressure.
Altitude barely matters at 10 m. At 2,000 m the air pressure is about 0.2 bar lower and the air is a little thinner, so there is slightly less drag. Over 10 m both effects are too small to see.
Recoil
A 0.5 g pellet at 150 m/s carries a tiny amount of momentum. On its own it would push a 1 kg pistol back at less than a tenth of a metre per second. But the air blast at the muzzle and the hammer hitting the valve inside add their own jolts. Makers fight them in different ways: an eight-port compensator and barrel vents on the Steyr Evo 10, and recoil absorbers on the FWB P8X, Walther LP500 and Pardini K12.
Think first: Which of these could you legally fit to a 10 m air pistol?
- A ported compensator that vents the air blast at the muzzle
- A device that actively steadies the pistol while you aim
- Both, as long as the pistol stays within the weight and box limits
No system may actively reduce pistol movement before the shot. Ported barrels and perforated attachments are permitted at 10 m.
- The regulator gives a constant charge while the cylinder stays above its set pressure (about bar). Below that, shots slow down and go low.
- Gravity pulls a 150 m/s pellet down about 22 mm over 10 m. Each 1 m/s of speed difference moves the impact about 0.3 mm.
- Know your shots per fill. Start every final with a full cylinder: 24 shots plus sighters.
- Temperature matters mainly when filling. Altitude is negligible at 10 m.
- Compensators and recoil absorbers are legal. Anything that steadies the pistol before the shot is not.
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