M100 shell (Power Down Shell) is designed to lower the muzzle velocity of the revolver from 120-140 m/s (0.20g BB) to 100 m/s. Often the muzzle velocity drops below 100 m/s, but it depends on the revolver and especially the length of the barrel.
In order to reduce the muzzle velocity, the amount of gas is limited with a smaller hole. In practice this means that if you feel the muzzle velocity is too slow, you can increase it by drilling the hole bigger.
Note! Contrary to the picture, the color of the shell is lighter, yellowish.