Machine Gunner

Machine-gunner, often called "MG", is mostly used for AA. It's also very cheap and small.

Main use
Machine gunners are used as Anti-Air, which means destroying incoming mortars/rockets/missiles projectiles. They shoot down all those projectiles automatically, but is somewhat inacurrate. Also, if it is fired too fast, it will overheat and will need to cool down before refiring.

They can also be used to destroy enemy's exposed weapons, mostly enemy machine gunners. The process of keeping the anti-air on your side of the map is called "Map control", and is key to winning matches.

MGs can be upgraded into Miniguns, that are better at dealing damage.

Other uses
They are used in Map control battles : you want to destroy your enemy's anti-air while having your anti-air up so they can't shoot mortars/rockets/missiles at you, since those are powerful weapons. Keeping map control requires practice and can change a match's outcome, especially amongst top players.

On a similar note, when defending attacks using MGs, while most players let them fire automatically, top players aim them manually (at least 3 of them in a group), since it's much more accurate, and allows them to preshot.

Also, top players sell the starting MG (the one which is already built on most forts) to get a few resources (some metal) which gives an edge when rushing.

Trivia
 * The machine gunner resembles the PM M1910
 * The machine gunners barrel would indicate that its water cooled yet in game it shows no such thing
 * this is most likely due to it looking cooler over realism.