There are 5 weapon slots for a soldier type unit
primary (eg M16)
secondary (eg LAW)
ammo (eg magazines, flares or grenades)
handgun + mags (eg Glock)
optics (eg binoculars)
Some types of unit have all four. Some, like a medic, sniper or civilian do not have a secondary weapon slot.
In general each weapon can fit into only one slot. Some weapons, like the M60 and PK, take up both primary and secondary slots. Units without a secondary slot cannot carry them.
Hope that helps.