Equipped with the M240B, the Machine Gunner has a higher rate of attack and deals more damage than the Rifleman at the expense of reduced accuracy.
Basically the commando of your army, they can make easy work of ANY infantry unit and light armored vehicles. But their short range makes them vulnerable against long range units and ineffective against turrets.
With their rapid-fire capability, a squad can make quick raids on the vulnerable parts of your enemy's base.
Mix them with your base defenders to give the enemy a hard time and a lot of casualties.