Most Drills are actually idiots, and all the trainers everywhere else make fun of them.

That being said, he was trying to point out something that kind of makes sense:

The 5.56 round has a propensity to tumble aggressively when it hits flesh. The logic of the entire round is that it's moving so fast that it has limited anti-armor capabilities, but when it hits flesh it will tumble and cause damage, making it basically an anti-armor and anti-personnel round. It's propensity to tumble in the body is what causes those massive secondary wound channels.

However, if you give it added anti-armor capabilities (like a tungsten core), and/or make it go faster, it might not slow down: thus preventing it from tumbling, and defeating the entire purpose of having a very high speed round that tumbles on contact.