Who are the guys kicking down the doors in iraq rifle men of machine gunners?

Generally, the machine-gunners set up in over-watch positions and pull security on the approaches during clearing operations. The guys who do the clearing can carry almost any weapon system, but the number one man usually carries a rifle. Having said that, it is a squad mission, and as such the whole squad works together: Riflemen, Grenadiers, Automatic Riflemen.