(U.S. Army) FM 3-22.3 states 36 rounds
The basic load for a M249 light machine gun is typically 200 rounds of 5.56x45mm NATO ammunition. This is usually carried in the form of linked belts or disintegrating link belts, allowing for sustained fire in combat situations. Soldiers may carry additional ammunition in pouches or packs, but the standard load is designed for immediate engagement.
600 to 800 rounds
the army base
Combat Basic Load for the M16 and M4 is 210 rounds. As the M4A1 is typically issued only to select units (Army Rangers, AF Para Rescue, etc.), personnel assigned this weapon would be likely to carry more than the basic load.
They don't carry ammo in boxes. Basic load depends on the weapon system. 210 rounds for the M4 or M16, 600 - 800 rounds for the M249, 800 - 1000 rounds for the M240.
The M249 Squad Automatic Weapon fires the same 5.56x45mm cartridge used by the M16 and M4.
In some countries which field it, it's operated as a crew served weapon. However, in the US military, it is fielded as an individual weapon.
The maximum effective range of the M249 light machine gun is 1,000 yards (910 meters).
buckle means bending
CLP
FN Herstal
They could be assigned an M16A2, M16A4, M4, M249, M240B, they could be put on an M2... depends on rank, position within their platoon, etc.