the .223/5.56 is designed to fragment when it contacts something. So, it causes a big wound channel, but has issues carrying past whatever it first hits, which is why the 9mm has more penetration in a home defense situation. Chalk it up to strange but true. The .223 is also a very light weight projectile in comparison to the typical 147gr JHP 9mm's that people load for HD. 55-77gr projectile that is designed to fragment, vs a 147gr projectile that is not.
Cliffs: .223 Tumbles once it gets out of wack in flight, and also fragments, so it has issues going through walls. Not saying a 9mm is a huge penetration risk, but it is much greater than the risk of a .223
Scoooot: If it is fairy dust, why is it pretty much standard fact in the gun community? And it's not so much the fragmentation, its the fact that the .223 tumbles after initial impact, also part of its design.