The Fabrique Nationale M249 is the US Army's standard-issue squad automatic weapon (SAW/light machine gun). It fires the 5.56x45mm NATO cartridge, and can accept either linked ammo or M-16
compatible magazines. It weighs 23 lbs loaded, 20 empty. It's intended for suppressive fire, not accurate point fire, and therefore comes equipped with a built-in bipod for spraying Charlies at long range. Standard US military doctrine is to issue one M249 for every 4-man fire team (two or three M249s per squad).
The M249 Para variant features a folding stock and a shorter barrel. It is designed for paratroopers, who naturally prefer its compact dimensions. However, the Para is most often spotted in the hands of hyperactive 13-year-olds playing that Counter-Strike
Corporal, I see dirty terrorists coming up the street. Keep their heads down with your SAW while I go around back and flank em with the rest of the squad.