5.56 mm is .22 caliber. While AR-15s can be purchased in those other rounds, they're uncommon. My assumption is that it was the typical .22 caliber, just as is the M-16.

There are lot of good reasons to use .22 for military use, including being able to pack more rounds in a can, and that wounding an enemy takes 5 guys out of service (they have to pack him away from the front and get him to medical care), while killing one only takes out one.

Shooting someone with armor piercing rounds leaves a neat hole poked through them, which is orders of magnitude less lethal than shooting someone with a round designed to mushroom and expend it's energy immediately.

I'll watch the video now, but a round that will penetrate to that depth of phone books is really unsuitable for causing maximum harm to the target. Also, I'm assuming the phone books are dry, which makes them much more easily penetrated than organic material.

Wet phone books are very close to the density of meat. This is why they are often used in bullet performance tests.

I'm displaying my assumptions a lot here =p

Imma shut up now and watch the video.

Thanks!

I have watched the video, and the assumptions I made are correct. The phone books are dry, and it was a full metal jacketed round, which didn't mushroom or deform at all.

If you look at the books the hole gradually gets larger as the bullet drags material from the previous book, or tumbles, into each new book.

A soft point hunting round would not have penetrated 5 wet phone books, and would have left a hole the size of your fist out the back of the first book. They are designed to mushroom, to spread wider, and to deliver the energy into the target, which is the opposite of full metal jacketed rounds, which are designed to penetrate armor and wound.