M85S Main spring 175mm x 1.2mm

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The main spring pushes the piston forward to pump high pressure air to fire the gel ball projectile.  This spring is longer, has one compressed section in the middle and rated at M85 that greatly improves FPS. This is a great step-up from the M75S main spring modification as it will give you more power and should be used in conjunction with a basic metal gear set.