Before you start tuning ...

Before you start, make sure you are familiar with "Before you start tuning" page. Below you can see also link to my tuning parts shop.

Air rifle powered by CO2 12g cartridge

Now let's consider an air rifle powered by a CO2 12g cartridge and test it.

Test environment:

• I use PCP conversion with adjustable pressure from 650-1050 psi (using 0 - 1500 psi pressure gauge). It simulates CO2 powered rifle in temperatures from 10-30℃.
• 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:

In temperature range 10-20℃: Factory spring works the best without spacer.

In temperature range 20-25℃: All spacers work similarly according to power criteria.

In temperature range 25-30℃: 12mm spacer gives a 5,5% power increment. The 4mm and 8mm spacer gives about a 4% power increment.

Factory spring is not bad. 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. Definitely, there is no need for a harder hammer spring. You can try to add small spacer during hot day. After all, there are more disadvantages than advantages, I'm not recommending this mod for CO2 air rifle.

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:

In temperature range 10-30℃: Super soft valve return spring performs the best. It gives 3,5-10% extra power.

Good choice for tuning, gives extra power in all temperatures. It sounds like a much better upgrade than a stronger hammer spring, because it doesn't influence on 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 even pellet may partially fall into the port and block there. Some people say you shouldn't drill it more than 3,8mm for a 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've 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:

In temperature range 10-30℃: When you change a 2,5mm transfer port to a bigger one, you will gain about 4% extra power.
When you change it from 3,5mm to 4mm without drilling barrel port and valve port it gives much smaller power gain, mostly during hot days.

Good choice for tuning when your rifle uses a 2,5mm transfer port. When you already have a 3,5mm transfer port, it gives very little profit. No disadvantages in air rifle handling. After drilling the barrel port and valve, there should be bigger power improvement.

Single upgrades comparison

Now you know how single upgrade works, below you can see a comparison between single upgrades to see which one gives the biggest power gain:

As you see, the best single upgrade is a super soft valve return spring. The second 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 a little extra power during hot days, but also restricts some power during cold days.

Multi upgrades comparison

Below you can see comparison between multi upgrades to see which combination gives the best results:

Those results are a little confusing. As you see, you can't add two upgrades together and expect double power gain from those upgrades. While upgrading air gun powered by CO2 cartridge, you achieve the best result while replacing valve return spring only. Replacing more parts like: transfer port (without drilling barrel/valve!), adding a stronger hammer spring or adding a spacer to factory spring gives nothing more after replacing valve return spring to the super soft one.

Do you need more power?

You can't do more with an air rifle powered by a CO2 12g capsule. To make it more powerful, you need to make external tank conversion. ... (as far as I know).