well bob, in real life terms there are 4 ways to fire the m249 saw effectively in combat.
prone is of course the most accurate because your bipod is down and your body is braced against the weapon.
firing from the hip is the second most accurate because you brace it against a meaty part of your body (the hip) when firing.
standing and kneeling are about the same in accuracy because neither provides a good stable balance to stop the recoil of the weapon.
so ....yeah, that's how it is in real life. I get to do it all once a month or so when we go to quantico and play at mout town.
i dunno why i do it. i think it's the fact that i am the one who ends someone else's life that excites me. kinda like playing god without any responsibility.