That's right it's a 6mm cartridge - the whole thing was designed to work as a system - rather than using 5.56 from a barrel length that stops it from doing it's job properly, they designed the cartridge and the gun to work together. For example, the cartridge is designed to burn all of it's propellant in the barrel to reduce flash and blast, and the bullet is specifically designed to function in the velocity range available from the muzzle to the maximum effective range. That's design done right. The last time anybody really made an effort to do this was the 1911 pistol.