Originally bought the OOD 5kg spring which is phenomenal and shoots approx. 150-160 fps with brass breech. Those results were verified with the chronobarrel so I should stress the word approximate. However, I'm not able to get a reading using this spring. The chronobarrel counts the darts down, but can't calculate the dart speed. They are FAST! The darts can punch through cardboard boxes at 10ft and dent PLA targets at the same distance. I'm afraid I will break the longshot if I try to prime it with one hand. The prime is tough, but the stock spring still holds the catch. Stock bolt sled but has held up so far. Wouldn't recommend this for indoor or close quarters play. You will hurt someone without a doubt!