The thing that usually forgets to get mentioned in the .223 / 5.56 MM comparison, is the fact the difference between the two is in the chamber dimensions, not the cartridge itself. The 5.56 MM chamber has more freebore than the standard .223 chamber does. Yes, the 5.56 case is slightly thicker, but that can be said for most all military cases. All the freebore accomplishes is it allows the round to be loaded slightly hotter for more velocity, without increasing pressure to dangerous levels. Weatherby has done the same thing for years in all of their Weatherby Magnum calibers.
This all means very little today simply because most all ammo today has been downloaded because of this interchangeability. So it is "protection" for a non existent condition. For example Weatherby calibers are now being chambered in non Weatherby makes like Remington. They don't freebore their rifles like Weatherby does, so to safely compensate for this they download the ammunition so it is safe to fire in any chamber. This also holds true for the .223 / 5.56 MM. While the 5.56 MM might be a little hotter, it isn't enough to create any danger. A lot of this is due to the "advancement" of the legal profession. In other words we have too many lawyers in this country. Compare a loading manual from the 50's or 60's to a manual copyrighted today and you will see what I mean. All of the "maximum loads" are much less in todays manuals. Bottom line, you won't get into any trouble shooting 5.56 in a .223, or 7.62 NATO in a .308. I've been doing it for years, and have never seen any issues of high pressure in either caliber in literally dozens of rifles. Bill T.