Originally Posted by alsaqr
.223 Remington cartridge case dimensions are identical to 5.56mm case dimensions. The original .223 chamber has a shorter leade than the 5.56mm chamber. A gunsmith friend tells me that no one currently makes a gun with a .223 SAAMI chamber. In any event there are 10-12 different .223/5.56mm chambers.
AR15BARRELS.COM - Technical Documents
It's a myth that 5.56mm US military cases are thicker than .223 cases. It's also a myth that the 5.56mm develops more pressure than the .223.
See Army TM 43-0001-27, chapter 10.
You got that right, but you can`t argue with some of these people, I even called Lake City which is less than a 100 miles from me & they said there is no difference .223 & 5.56 NATO come of the same line, but O well believe as wish, but some of us know better.