Quality durable spring. Used with a ML 6.04×510mm barrel and the TTi One piece 7mm spring guide inside the AAC twisted cylinder, it gives 384 Fps @0.25g or 340 Fps @0.32g, so 1.7 Joules... There is more powerful, as the PDi 450 with 9mm guide, but it is well calibrated and offers good result with a decent Hop-up.