Quote Originally Posted by Jetpig View Post
There is NO difference in 556/223 brass. The only difference is in factory ammo. Anything labeled 556 is either loaded to higher 556 pressure or manufactured in a metric country or both. When you reload 556 brass using 223 data, you are loading 223. There is data available to load to 556 pressures. Most bolt rifles have no issues firing 556 pressure. Any firearm you shoot a true 556 out of will produce a flat primer but there is much more to identifying pressure signs than the primer. Same general rule always applies.
Start mid-range and work up. The standard reloading books are conservative on data. I have had this discussion with Sierra on more than one occasion. The powder manufacturers are a bit more liberal with their data.
Enjoy!
That's not completely accurate, especially when dealing with Mil-Spec 556 brass. 556 brass (and especially Mil-Spec stuff) has a thicker wall than 223 specs, which can create less volume and higher pressure.