The following units can be upgraded to Machine Guns:
Machine Guns are powerful defensive weapons, especially against enemy gunpowder units. These units cannot initiate attacks, making them most useful when defending cities.
Many WWII-era divisions had a heavy weapons unit composed of a number of heavy machine guns, and perhaps a battery of 8cm field mortars. While such units were sometimes attached on the company level to a battalion, there were also entire battalions of machineguns attached at the division or corps level. The defensive ability of massed machineguns was first illustrated with deadly earnest during WWI by windrows of dead. By WWII the typical machinegun's rate of fire and increased and its bulk decreased, thus making it even deadlier and more mobile.