Depending on what power range your using should be considered. If your throwing them out a standard gamo, go with the lights, the heavies might strain the spring and decrease it's life span. (CPL's shoot great in my S1K) Guns such as the RWS 350 or a Webley Patriot (really pack an umph) shoot the heavies rather well. They increase FPE on these stronger guns as well as when pellet speed decreases in this monsters with a heavier pellet, they tend to group better. Stick with the lights in a gamo, UNLESS you have a Hunter extreme/Hurricane. HTH