Classic Army's Version 2, 7mm Type gearbox differs from other standard 6mm gearboxes in that the bushing holes are larger. These gearboxes are primarily designed for high-speed use in MP5's, however can also fit all MP5 (Except MP5K and PDW), M16, and G3 Series airsoft rifles. Each gearbox kit includes a complete set of 6 ball bearing bushings, which decrease friction and increase gearbox speed. This is recommended for use for high-speed, low velocity applications. Bearings are not designed for use in high-torque airsoft guns. Feel free to contact us with any questions you have prior to purchasing this product.