These JSB Exact King Heavy are the heavier .25 caliber domed pellets requested by many air-gunners that have more powerful PCP air rifles. These guys can do some amazing things at longer distances when used in rifles that can get them up to speed. They have a wide skirt so if you are having loading issues switch over to the MK II version of these which have thinner skirts specifically for this reason both are amazing pellets.
It's all a balance between power, weight and twist rate. If you get those right then you will see the results!