These Dart Zone Pro Mk-4 Springs are compatible springs to tune the performance of the DZP Mk-4. The Dart Zone Pro MK-4 may have some puzzling design decisions, but with its quick-swap spring system and gasketed barrel extension, It's one of the easiest to tune up and down. FEATURES:• Dart Zone Pro Mk-4 Spring• Tune your blaster up or down to suit any event• Easy, near-toolless install• Reliable, consistent performance• Compatible with the DZP Mk-4 and Nexus Pro X INCLUDES:• 1 DZP Mk-4 Upgrade Spring PERFORMANCE:Average FPS without the FPS+ Barrel* • 130 FPS with the OOD 1.2 x 350mm Spring• 193 FPS with the OOD 1.4 x 350mm Spring• 222 FPS with the OOD 1.5 x 350mm Spring• 238 FPS with the OOD 1.6 x 350mm Spring• 264 FPS with the OOD 1.8 x 350mm Spring• 275 FPS with the OOD 1.9 x 350mm Spring (Experimental, may take a set over time) Average FPS with the FPS+ Barrel* • 311 FPS with the OOD 1.9 x 350mm Spring (Experimental, may take a set over time) *These performance averages are a ballpark measurement. The standard deviation is not listed here because we suspect our blaster needs lubrication and maintenance.  SPECS:• Material: Spring Steel• Dimensions:      Gauges: 1.2-1.9mm      Length: 350mm     Outer Diameter: 14.6mm LINKS:• More Springs