SHS 6mm Metal Airsoft Bushings AEG Gearbox Steel Bushings
When you purchase an AEG, most likely it comes with platic bushings.  Those plastic bushings break easily.  Techs highly recommended you to upgrade your standard plastic bushings to solid metal airsoft bushings. SHS bushings can take all the stress your setup throws at it.  Unlike ball bearing bushings, they won't break under high strain.