This can be done by modifying the spring guide so that the installed height of the spring is reduced. You can also make the "top hat" longer or add spacers between the "top hat" and piston.
There are limitations to reducing the springs installed height that should be considered. Possible coil bind when the spring is compressed can result in the trigger not engaging the piston. A harsher firing cycle can also result. Most of the methods described also have the potential to cause the piston to bind before the trigger can engage it.
How effective this is in increasing velocity is dependent on a few factors as well. Some springs will simply "set" to the new installed length which reduces the power to the same level present before the modification, possibly lower. Pellet weight, fit, and the size of the transfer port also play roles. To explain those would take a fair amount of time.
Considering the increased wear and tear as well as the harsh firing cycle that can result, this gun does not seem like a good candidate for this modification. It has been shown that a simple "lube tune" by itself can raise velocity by 50fps alone. Combine that with a gun that is known for exceeding it's advertised velocity and you have a smooth and powerful tool.
Just my opinion.
Russ S.
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