This spring is specifically designed for use in AEP & CMG series from Tokyo Marui and is probably the most powerful micro spring available.

There are two versions of this spring available, the M75 and the M80. 
We generally recommend the M75 for AEPs (Automatic Electric Pistols) unless the motor is also upgraded, and the M80 for use in CMGs (Compact Machine Guns). 

Due to the additional strength compared to the stock springs, the trigger response and RoF will decrease and the power draw will increase. 
We recommend only using these springs when paired with a high-discharge LiPo and ideally, an upgraded motor.