The performance of your weapon is one of the main factors responsible for combat effectiveness. The compression and stability of the mechanism depends on the details of the gearbox of the AEG, therefore, especially with high tuning, it is better to replace the walls with more durable ones.   

These Gearbox walls from Bull Gear are suitable for M4 and M16 airsoft guns. These walls have greater strength, they will withstand any tune and provide better fixation of the entire mechanism. The Spring Guide (included) provides quick spring replacement without disassembling the Gearbox. The Gearbox walls are protected by anodizing.   

The Gearbox walls are made of a single bar of Aluminum D16T, by milling on a high-precision CNC machine.