This Airsoft replica features the ESA Quick Spring Change System. Rapidly and quickly adapt your FPS without needing access to a workshop or a set of special tools; just remove the shoulder rest top in order to gain access to the spring so that it can be removed without removing the gearbox. The replica is fully compatible with LiPo and LiFe batteries. This replica is perfect for operating on a medium range due to the base FPS of ~380 FPS provided by the main default spring. This set includes an M90 spring which reduces the FPS to ~310 FPS which allows you to play in most indoor Airsoft games. The gearbox contains an 8mm top bearings as well as shimmed steel gears. The system is secured with 4 anti-reversal latches. A light polymer piston equipped with full steel strip. Also installed are a type 0-cyl, a steel circuit-breaker, and a precise rotary Hop-up chamber. The hop-up perfectly fits with a high quality 6.03 precision-barrol. Length: 640mm - 720mm (Adjustable) Weight: 2150g Mag: 380rd Hi-Capacity. Works with Matrix, Tokyo Marui and other compatible Airsoft M4 Series AEG Airsoft Mags Velocity: 310~380 FPS (w/ 0.20g BBs) Thread: 14mm Negative Gearbox: Ver 2 Metal, Fully Upgradeable Motor: Long Type Modes: Semi/Full-Auto, Safety Battery: 7.4v / 11.1v Buffer Tube LiPo Battery Recommended (Wired to rear w/ Deans connector. Battery and Charger not included. We recommend using only high quality batteries and smart chargers) Hopup: Yes, Adjustable