Think about it carefully, muzzle energy isn't a variable, but a solution to an equation.
Muzzle energy is a product of a specific volume of compressed air pushing a spheroid of a given weight down a length of tube. You can directly alter any of those variables, but you can't make the gun lower it's muzzle energy without changing any of those variables.
This is an area that crosses over into firearm physics.
The simplest way to increase the muzzle velocity of a bullet, is to make the barrel longer. The bullet spends more time under pressure from expanding gasses, and therefore leaves the barrel faster.
Compare the MC51 to a G3A3, the MC51 has 800J less muzzle energy due to the shorter barrel.