I consulted mainly two sources: (i) Dslyecxi's Tactics, Techniques and Procedures Guide for ArmA 2, and (ii) FMFM 6-5, Marine Rifle Squad, the doctrinal publication available elsewhere in the web.
- Mission of the USMC Rifle Squad:
The mission of the Marine Rifle Squad is to locate, close with, and destroy the enemy by fire and maneuver, or to repel the enemy's assault by fire and close combat.Note those are the missions of the squad, not the fire team. The mission of the fire team is to execute the squad leader's orders, which are issued with the squad's tactical mission in mind. Nonetheless, note the words "close with" and "fire and maneuver" in the quote above. Eventually, are we supposed to move out from the security of cover until we see the "white of the eyes" of the enemy? Euphemisms apart, this scares the hell out of me.
The USMC fire team is composed of the following personnel.

Fire Team Leader (FTL)
-Executes the orders imparted by the squad leader
-Positions himself where he can control the fire team
-Controls the fire discipline of the fire team
-Exercises fire control over the fire team
-Positions himself near the automatic rifleman so he can effectively control his fire
-He is also the fire team grenadier, using the M203 grenade launcher to cover the dead space/s in the principal direction of fire (PDF)

Automatic Rifleman (AR)
-Under the direct control of the fire team leader (FTL)
-Responsible for the squad automatic weapon (SAW)
-The automatic rifleman carries the weapon with the highest firepower and rate of fire in the fire team
-The foundation and bedrock of the ability that the fire team has to destroy the enemy. Ignore this guy at your own cost and peril.
-Assumes the role of fire team leader if the later becomes a casualty

Assistant Automatic Rifleman (AAR)
-Primarily a rifleman, he also carries extra barrels and ammunition for the SAW
-Trained to assume the role of automatic rifleman
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Rifleman (R)
-The rifleman is usually trained in the point man and scout roles
To be continued ...
Cheers,