Many years ago, I'm talking early 20th century, most soldiers didn't even fire their weapons. Or if they did, they would just aim over the heads of the enemies. They did it on purpose. British Army. World War I. The brigadier, he'd walk the line with a stick and he'd whack his men in order to get them to shoot. Even in World War II, in a firefight, only 15%, 20% of the men would pull the trigger. The fate of the world at stake and only 15% of them fired.
Now, what does that tell you? It tells me that that war would have been over a whole lot quicker had the military got its shit together. So we adapted. Better training. Better conditioning. Then comes the Vietnam War, and the shooting percentage goes up to 85. Lots of bullets flying. The kills were still low. Plus the guys who did get a kill, well, most of them came back all messed up in the head.
This question should probably be in the FAQs somewhere...
Anyways, the show didn't make this up, but it's based on controversial research.
Here in a previous answer /u/Georgy_K_Zhukov lays out the historiography of this idea.