If you need automatic fire for long period of times then you need a machine gun. A machine gun can give constant covering of fire from 10 seconds and up to a few minutes. This ability forces the enemy soldiers to take cover and stick to their places. In the First World War the machine guns caused the largest amount of causalities in the battle field. The machine gun was very efficient in breaking the assembly lines of troopers. Beside that it can supply suppressive fire which allows your own troops to move and improve positions while the enemy is suppressed. It can help hit fast moving targets, clean dangerous areas in a method that is called spray and pray and of course in defending a position in a very efficient way.
The fire rate usually moves between 700-1000 rounds per minute, just think of it, it is so amazing. Just think of it. Most assult rifles today can carry 30 bullets stacks and if they’ll do automatic fire they’ll finish the ammo very fast. Also their accuracy and the heating up of the barrel can make the assault rifles usless in these kind of shooting rates. In a machine gun you usually carry the bullets in a drum and in chains of bullets. Also the barrels are thicker and are cooled down in order to reduce the heating effect. Usually you use a machine gun lying on the ground when the gun is set on a tripod or bipod.