Every unit has a prescribed TOE, table of organization and equipment. For instance, a platoon is generally made up of four squads. Depending upon what type of unit it is, the TOE varies. A standard infantry squad will have a squad machine gun, and one or tow grenade launchers. Everyone else carries an assault rifle and grenades.

At the platoon level there is generally one squad designated as the weapons squad. They carry heavier stuff like multiple MGs, light mortars, etc...

As troops fit out for any given operation, they'll vary the arms they carry based upon the mission, weight of the gear, and such. A rifleman has his M-16 and a couple hundred rounds, maybe 10 grenades at most as his basic kit. Maybe he's carrying a claymore mine, as well, maybe a LAW, maybe an extra belt for the M-60...it all depends on the mission and is different every time.

Once you get past the basic load, the mission determines what you carry, and the squad leader and platoon sergeant are responsible for telling troops what they're going to carry. There is a little leeway given for seasoned troops who want to carry, say, more grenades or a pistol, but most of it is proscribed by the NCOs.