These JSB Jumbo Exact Heavy were made for the higher-powered rifles, like your magnum break barrels or PCPs. These give those higher-powered rifles the ability to really shine. When you have too much power for lighter pellets they will travel at a high velocity and speed isn't always be a good thing. When a pellet travels too fast it can cause issues with accuracy and if you're getting into supersonic speeds it can also make things pretty loud.
It’s all a balance between power, weight and twist rate. If you get those right then you will see the results!