M170 AEG Spring Japanese Black Steel. One of our most popular airsoft upgrade springs! Very high quality steel made spring to get your gun running a high FPS. Fits in all standard gearboxes. This is your standard upgrade spring strength for upgrades.   This is a non-linear spring utilizing Japanese made steel spring material. Black color to differentiate spring types - Comparable to SHS TH1030 - Alloy steel construction - Tempered Si-Cr - Sturdy and Balanced - For Use with Various AEGs