5.56 nato vs 5.56 x45?

5.56×45 NATO – 4 Examples

There is no difference between 5.56x45mm and 5.56 NATO. They are the exact same cartridge, just using a slightly different name. The former is simply a dimensional description, while the latter, 5.56 NATO, is the official nomenclature.

5.56×45 NATO vs .223 Rem How To Determine Your Rifle Chamber