The Core Infantry Handbook
First and foremost everyone is an infantryman, but beyond specialised roles there are core infantry roles common throughout all fireteams, squads and platoons.
The assistant automatic rifleman, or "AAR", is the right-hand man of the automatic rifleman. They help spread-load the ammunition duties with the AR by carrying additional ammunition for that weapon. The AAR's role is to stick with the AR and provide support - the two always form a buddy team. The AAR supports the AR in the form of providing security, helping to spot, engage, and adjust fire on targets.
The automatic rifleman is the fireteam's heavy firepower, giving them the ability to throw hundreds of rounds downrange in short order. The AR is responsible for employing their weapon in a manner that maximizes the killing and suppressive power of it, allowing their teammates to maneuver with the support of their fire.
A Medium Machinegunner or MMG has the ability to place sustained accurate fire in high volume on the enemy is capable of inflicting a large number of casualties in short order when properly employed. Medium machineguns typically fire a 7.62x51mm caliber bullet or larger - significantly more powerful than the infantry's normal rounds. An MMG has a longer range than an automatic rifle, and by default are loaded to fire daylight-visible tracer ammunition every fifth round. When employed in a base-of-fire or support-by-fire position, or when employed in the defense, MMGs are a powerful asset to any unit.
The Heavy Machinegunner or HMG is the senior member of a heavy machinegun team. He carries and employs a .50 calibre weapon system, Heavy machineguns give infantry a tremendous range and a powerful punch - capable of defeating light armored vehicles as well as punching through heavy cover. The HMG chooses where to employ the weapon and is responsible for relocating as required, in coordination with higher leadership. A HMG is usually supported by multiple assistants.