.223 5.56 difference NRA

This NRA Article Describes the Difference Between 5.56 NATO and .223 Remington

If you’ve ever wondered about the differences between 5.56 NATO and .223 Remington, well, this article gives a good account of the differences.

Bottom line is, if you’ve a .223 rifle, do not fire 5.56mm from it.  If you’ve a 5.56mm rifle, you’re OK (but may see some inaccuracy due to the added “slop”, the slop being an allowance to ensure the firearm will fire when dirty/grimy).