The JSB JUMBO MONSTER PELLETS are easily the heaviest pellet produced by JSB. It's weight is fixated to match the power of most PCP Rifles, and despite the massive weight, it offers a stable flight for marksman who seek maximum stability in normal and extreme weather and wind conditions. The workmanship behind the ''MONSTER'' pellets come from specially designed weights and shapes that result in a perfectly distributed and aligned center of gravity. 

SPECIFICATIONS:
Pellet Diameter: 5.52mm
Weight: 25.39gr 
Shape: Rounded head

 

Please have a look here;

and for those a little more technically inclined;