I won't get into this in any great detail, but I am responsible for the firearms training for my division of the state police. Let's put this rumor to rest. Here is a sketch of reality, if you want/need more information, it is widely available on the internet.
Most specialized military and law enforcement 5.56/223 ammo is designed to penetrate several inches of tissue; the bullet then yaws (turns sideways). When this happens, the projectile fragments, usuall at the cannelure first (Yes, even the SS109 projectile)because of the incredible velocity and resistance that is placed upon it. Try to think of the physics of the heavy bullet base wanting to overtake the lighter weight nose of the bullet, due mainly to disruption of the spin (which is what is stabilizing the bullet). The fragments take seperate paths in their travel through tissue. Ballisticians and forensic pathologists all agree that this is a significant wounding factor, seen in this caliber.
The above facts are, and have been well documented by the U.S. Army Wound Ballistics Laboratory (ie. Dr. Martin Fackler, MD), leading pathologists (Dr. Vincent DiMaio, MD), and the Firearms Training Unit (FBI, Quantico, Va.), as well as "your's truly" when testing .223 rounds in calibrated ballistic gelation. This was first seen in soldiers shot in Vietnam by captured M16's, with the M193 (55 gr. Ball). It's a phenomenon distinctive of this velocity/projectile weight, ie. the specific round.
The problems arise when people read this material, don't understand it, then start making outlandish posts on the internet. The rumors start flying, then it's cast in stone as factual.
If you want additional information, go to:
www.firearmstactical.com or
the International Wound Ballsitics Association (IWBA).
I hope that my comments are a helpful lead to those who are interested in putting this silly "tumbling bullet" fairy tale to bed. My explanations are not intended to by scientific, but are the best that this dumb cop can do.