Muzzle energy is defined as the bullet's kinetic energy upon exit, which is proportional to the square of velocity, which means barrel length necessarily affect

Muzzle energy is defined as the bullet's kinetic energy upon exit, which is proportional to the square of velocity, which means barrel length necessarily affects muzzle energy more than muzzle velocity. From quick Google searches, 9mm pistol barrels are around 4-5 inches long, 9mm rifles around 16". This has a major impact on muzzle velocity, and an even greater impact on muzzle energy.