These springs are build after careful study of major Japanese spring sets providing all the features you'll look for in an AEG spring for maximum performance. They are made out of high quality anodized piano wire, with tension carefully measured. Systema's irregular pitch, Prometheus's Stainless steel material and PDI's durability are all features duplicated and found in a these springs. Features:Irregular pitch spring coils: Variable tension. Reduces stress, wear and tear for your gearboxStrength: Made out of stainless steel. Increased tension and durability. Tension lasts 50% longer than other leading brandsCompatibility: Designed for all versions of Airsoft AEG gearboxesSpring Power Chart. (% / FPS)FPS estimate based on average power output. Exact FPS result may vary based on alignment, installation, inner barrel, cylinder and other factors.120% (M90) ~280~340 F.P.S.130% (M95) ~290~370 F.P.S.140% (M100) ~320~380 F.P.S.150% (M110) ~330~390 F.P.S.160% (M113) ~340~400F.P.S.170% (M115) ~350~420 F.P.S.190% (M120) ~360~450 F.P.S.210% (M125) ~400~470 F.P.S.235% (M130) ~420~500 F.P.S.260% (M140) ~450~550 F.P.S.300% (M150) ~480~600 F.P.S.M160 : ~530+ 20 F.P.S.M170 : ~560+ 20 F.P.S.M180 : ~595+ 20 F.P.S.M190 : ~625 + 20 F.P.S.