What is the range of a machine gun?

The "typical" maximum effective range of a 7.62mm NATO machinegun is approximately 700-1000 M depending of course on the abilities of the gunner. This can vary with the addition of optical sights and the mounting of the machinegun. If the MG is mounted on a tripod it will be more accurate than a shaky bipod mount. Heavier machineguns, .50 cal and 12.5mm etc have a longer effective range due to the higher propellant charge and stability of the larger projectiles over range. Also with the heavier machine guns they are often mounted in armored fighting vehicles (tanks and apc's) which will, more often than not, have optical sights and computerized ranging. This makes their effective range quite a lot more than an infantryman with his bipod mounted MG.

MG range is very situational and cannot really be tied down to one simple answer.