S.L.A. Marshall's study on soldiers' ability to fire on the enemy is from WWII and had been widely quoted. The "study", however was found to to be fraudulant and is now dismissed.
More recent studies from Nam indicate a large majority of troops in combat used their weapons. Many only did the "spray and pray" routine, but at least rounds went down range.
Information from the Gulf and Iraq wars indicated a larger percentage of troops are effective with their individual weapons.