The GSI Air Cocking Sniper Rifle 13mm V Spring uses oil tempered steel wire for maintaining spring elasticity for a long lifecycle. 
Possible to use a bearing guide thanks to the grinding process.
Corrosion prevention by ferric oxide film treatment
In the initial installation stage, there might be a 5% length variation; however, the elasticity will not be changed. 
Applicable 13mm spring guide:  Marui VSR-10, VFC ASC338 & M40A3, King Arms R93, MB Series when upgrading to 13mm, and DONGSAN Model M70 Series
Released with 5 different spring bands (please select desired one from the drop-down list): 
VS 130 - 430~440FT     EAN & SKU:  8809899600542, AS20-ASP130
VS 150 - 480~490FT     EAN & SKU: 8809899600559, AS20-ASP150
VS 160 - 520~530FT     EAN & SKU: 8809899600566,  AS20-ASP160
VS 180 - 600~610FT    EAN & SKU: 8809899600573,  AS20-ASP180
VS 200 - 630~650FT
Dimension & Weight:  8.75 (L) x 0.47 (W) x 0.47(W),  0.8oz