Hello,
With over twenty years of experience modifying airguns I turned my attention to the Umarex CP99 pistol and managed to reach a power of 7.5 joules !
To my knowledge, it is now the most powerful CO2 multi-shot pistol in .177 caliber. This result is the outcome of many years of fine-tuning and optimization. I tested a wide range of valve springs with different wire thicknesses, coil counts, and free lengths. To refine all my settings, I went through around 500+ CO2 cartridges during the tests phase, exploring every possible way to gain both velocity and shot-to-shot consistency. As a result, all internal parts have been modified.
Out of the box, the CP99 produces about 350 fps, which translates to around 2.5 to 3 joules depending on the pellet weight. Thanks to the modifications listed below and while keeping the original 3.35 inches barrel, I achieved a maximum of 445 fps with 0.45 g pellets roughly 4.2 joules. With the stock barrel, which is quite short, and with the same internal modifications, you’ll never exceed that : around 4 to 4.3 joules maximum. This is already decent, but it still wasn’t enough for me.
The limiting factor at that point was clearly the barrel length, so I installed a custom machined 7.5 inches barrel, partially hiden within a Weihrauch silencer. The sound suppression remains very effective. The improvement was immediate, accuracy became outstanding, and the power took a big leap. I was finally able to fully exploit all the internal work, reaching 560–570 fps with cheap 0.50 g pellets, which translates to around 7.5 joules.
I've modified :
-"Moon" valve with enlarged 4 mm output port and widened CO2 inlet
-Mag head fully machined to increase CO2 volume
-Spacers added under the CO2 piercing pin to increase volume
-Piercing pin bored out to 1.30 mm and filter removed
-Valve spring lightened and optimized
-Reinforced hammer spring (imported from Indonesia)
-Threaded steel insert in the alloy hammer for reinforcement and improved valve opening
-Improved sealing between barrel, cylinder, and CO2 outlet
-7.5 inches rifled barrel
FPS | Joules
567.7 | 7.49
569.0 | 7.52
567.4 | 7.48
566.4 | 7.46
564.1 | 7.40
548.4 | 7.00
520.5 | 6.29
Max : 7.52 J
Average : 7.23 j
With over twenty years of experience modifying airguns I turned my attention to the Umarex CP99 pistol and managed to reach a power of 7.5 joules !
To my knowledge, it is now the most powerful CO2 multi-shot pistol in .177 caliber. This result is the outcome of many years of fine-tuning and optimization. I tested a wide range of valve springs with different wire thicknesses, coil counts, and free lengths. To refine all my settings, I went through around 500+ CO2 cartridges during the tests phase, exploring every possible way to gain both velocity and shot-to-shot consistency. As a result, all internal parts have been modified.
Out of the box, the CP99 produces about 350 fps, which translates to around 2.5 to 3 joules depending on the pellet weight. Thanks to the modifications listed below and while keeping the original 3.35 inches barrel, I achieved a maximum of 445 fps with 0.45 g pellets roughly 4.2 joules. With the stock barrel, which is quite short, and with the same internal modifications, you’ll never exceed that : around 4 to 4.3 joules maximum. This is already decent, but it still wasn’t enough for me.
The limiting factor at that point was clearly the barrel length, so I installed a custom machined 7.5 inches barrel, partially hiden within a Weihrauch silencer. The sound suppression remains very effective. The improvement was immediate, accuracy became outstanding, and the power took a big leap. I was finally able to fully exploit all the internal work, reaching 560–570 fps with cheap 0.50 g pellets, which translates to around 7.5 joules.
I've modified :
-"Moon" valve with enlarged 4 mm output port and widened CO2 inlet
-Mag head fully machined to increase CO2 volume
-Spacers added under the CO2 piercing pin to increase volume
-Piercing pin bored out to 1.30 mm and filter removed
-Valve spring lightened and optimized
-Reinforced hammer spring (imported from Indonesia)
-Threaded steel insert in the alloy hammer for reinforcement and improved valve opening
-Improved sealing between barrel, cylinder, and CO2 outlet
-7.5 inches rifled barrel
Chrony test : https://streamable.com/l6mzn8
75.2 °F 0.50g pellet
FPS | Joules
567.7 | 7.49
569.0 | 7.52
567.4 | 7.48
566.4 | 7.46
564.1 | 7.40
548.4 | 7.00
520.5 | 6.29
Max : 7.52 J
Average : 7.23 j
Last edited:
