To the best of my knowledge, the reason the M16 is semi/burst is because during the Vietnam era the weapon was fully automatic. As it turns out, this made it wildly ineffective. They had to restrict the weapons to burst/semi because the soldiers using the weapons were annihilating their ammo supply more than the enemy.
This is also an amusing argument to claim that automatic weapons are safer than semi-automatic weapons, if you wanted to be cheeky about it.
Another amusing little aside, is
this gem of a situation. Which seems to suggest that even machineguns aren't sufficient for large game hunting.