First point to consider. The current firearms law (which was amended recently to include airsoft units, categorized under "low-power airguns") with its IRR, states that the maximum legal FPS for an airsoft rifle is 550 FPS using 0.20g BB's. If you plan to register your gun with FED, then you cannot have it registered if it goes over 550 FPS.
Second point, I believe there is no "stock" unit, whether "high end" like Tokyo Marui, or "low-end" like Jing Gong, that can reach 600 FPS as is (i.e. "fresh out of the box"). The average out of the box FPS for a Tokyo Marui unit is just around 300-350 FPS (M4A1 type), while nowadays, owing to pre-upgraded internals and newer factory gearbox modifications, Jing Gong units (made in China) reach from 400-450 FPS out of the box. The reason behind the low FPS of Tokyo Marui units is that, in Japan, they have a law that states that only 1-Joule (energy of BB impact) is allowed for airsoft units. 1-Joule corresponds to roughly about 340 FPS using a 0.20g BB.
Third point. All stock airsoft units, regardless if it's high end or low end, can indeed be made to fire 0.20g BB's in excess of 600 FPS, but this entails a lot of internal/gear box modifications and a skillful AEG gunsmith. The issue being solved here is proper sealing, and allowing more air to be compressed to propel the released BB faster, which can be achieved by the following:
a) changing of cylinder interior diameter (bore-up)
b) stiffer spring
c) Torque-up gear set
d) more efficient motor
e) high-end (i.e. Systema) sealing components, air nozzle, hop-up chamber and packing rubber
f) tight and long barrel (6.03)
e) high mAH (not voltage) battery
If you do all of the above changes (in the hands of a very experienced gunsmith), any airsoft unit, save for those with barrels shorter than a standard M4A1, can attain FPS of over 500 and can even reach 600 in the right conditions.
-RODION



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