Ahjae TV does a review of the INF M4 CQB that is available in South Korea... "It is a must for first-time buyers. 7.4V battery series is recommended for less than M90 Spring, and 11.1V is recommended for M100 Spring or more. Also, if you use a spring that is too strong to feel recoil, you need to upgrade the gear to torque-up (half-teeth piston is required when replacing this gear), etc. For the motor, we recommend a high-torque-up nadymium motor."