1. The reason why they expended 200 rounds during a firefight when they clearly couldn't see the enemy.
Grunts can be trigger happy. They enjoy firing their weapons at the bad guys, hoping to score a solid kill shot, even if it means expending 90% of their ammo. Half the time, the ground pounders don't get a clear line of sight on enemy movement from ground level.
But they still pull the trigger.
2. Why they shot so poorly at the range.
Not every infantryman is a crack shot. When you're competing for bragging rights throughout the platoon and you don't win, excuses are made.