Before you start tuning ...
Before you start, make sure you are familiar with "Before you start tuning" page. Below you can see a link to my tuning parts shop too.
Air rifle powered by external PCP tank
Now let's consider an air rifle powered by an external tank with compressed air. The main difference between CO2 external tank and PCP tank is the possibility to add pressure regulator. Finally, we are independent of variable CO2 pressure. We also can increase it ... a lot!
Test environment:
- I use PCP conversion with adjustable pressure from 1000-2400 psi
- During the test I used JSB Exact 4,50mm (0,547g).
- I used an Artemis CP2 air rifle in caliber 4,50mm with a long barrel.
- Barrel port and valve unit is not drilled yet to a bigger size (important while playing with the transfer port)!
Stronger hammer spring
I added various spacers (4, 8 and 12mm) to the factory spring. Bigger spring preload makes the hammer strike valve harder. How about power increase? Let's see the chart:
Factory spring is not bad. Adding a spacer makes this spring strong enough. A stronger hammer spring makes the air rifle harder to reload, the air rifle kicks harder (accuracy may decrease), and the trigger is harder to operate. In the chart you can see how the stronger spring allows you to open the valve holding bigger pressure. To achieve bigger power, it is the way to go. But don't use too big spacer - it is not nice to operate.
Piercer eliminator mod
A piercer limits air flow to the valve chamber, so removing it is a good idea. Below you can see a comparison between the factory air rifle and air rifle without the cartridge piercer:
As you see, we have nice power gain, just after removing the cartridge piercer. Without any disadvantages in air gun handling (trigger action, air rifle reloading force, kicking during shooting). Peak power is still in low-pressure zone, but we know how to fix it.
Weaker valve return spring
I tested various valve return springs (medium, soft, super soft) instead of factory valve return spring. With a weaker valve spring it is easier to open the valve, the valve stays open for more time because the weaker valve spring pushes the hammer back with smaller force. How about power increase? Let's see the chart:
Good choice for tuning, gives extra power, and it doesn't influence the trigger action, air rifle reloading force or kicking during shooting.
Transfer port size
As far as I know in a couple countries this air rifle is equipped with a 2,5mm transfer port and in fact it can restrict the power a little.
Keep in mind that the transfer port is only one part, there are two more: barrel port and valve port. To achieve full power gain, you should drill the barrel port and valve port to a bigger size.
Making the barrel port hole too big may damage pellet during loading, or pellet may partially fall into port and block there. Some people say you shouldn't drill it bigger than 3,8mm for 4,5mm caliber. During drilling, you can also damage the barrel and valve, drill press is recommended.
I plan some more tests with "factory" air rifle so my barrel and valve ports are factory size, about 3,5mm. I tested various transfer port sizes (port diameter 2,5mm; 3,0mm and 4,0mm) instead of the 3,5mm factory transfer port. A bigger transfer port restricts gas flow less, so bigger is better. How about power increase? Let's see the chart:
Changing transfer port to 4mm gives a little extra power when you use 3,5mm. Somehow transfer port in 2,5mm size performs better than 3,5mm and 4mm in some areas - no idea why. Later I'll add one more test with transfer port size.
Single upgrades comparison
Now you know how single upgrade works, below you can see a comparison between single upgrades on one chart:
The best single upgrade is the piercer eliminator mod. It gives nice peak power at low pressure zone. Second one is the softer valve spring, it gives some extra power between 1000 and 1350 psi. The third one is a 4mm transfer port which gives little boost even with the stock barrel and valve (after drilling the barrel/valve it should work better). Those upgrades have no disadvantages in air gun handling. On the last place is a hammer spring spacer which gives an extra power, but it influences the trigger action, air rifle reloading force or kicking during shooting.
Softer valve spring, piercer eliminator vs transfer port (one more time)
From single upgrades, I picked piercer eliminator mod and softer valve spring. And I checked those mods with various transfer port sizes:
Piercer eliminator mod and softer valve spring works very nice together. I think it is the way to go. As you see, the small transfer port restricts power ... as expected. 4mm transfer port produces a little less power than factory one, which is 3,5mm. I think it may be caused by the bigger volume of transfer port, so pressure inside the barrel is a little smaller. After drilling barrel port and valve port to bigger size, there should be improvement. Without drilling, changing transfer port may not be the best idea.
Softer valve spring, piercer eliminator vs hammer spring
I picked the piercer eliminator mod and the softer valve spring. I think those are the best mods (without drilling barrel and valve to bigger size). The only way to add more power is running it at higher pressure and adding spacers to the hammer spring:
Piercer eliminator mod and softer valve spring works very nice together. I think it is the way to go. Without drilling barrel port and valve port to bigger size, you can only add bigger spacer to factory hammer spring and increase pressure using regulator. I don't recommend more than 8mm hammer spring spacer. To make it easier to shoot, I recommend soft trigger spring and trigger set to one travel mode.
Testing heavier pellet?
I checked JSB Exact Monster pellet to see how it performs. All test before were performed with JSB Exact pellet.
With heavier pellet, you can gain some power on same hardware settings, so the air gun becomes more power efficient.
Do you need instruction to make external PCP tank build?
Instruction and parts list needed for external PCP tank build are available here:
