The 5.56mm NATO vs. the .223 Remington| 223 Remington vs 5.56mm NATO
Let’s start with the ubiquitous .223 Remington first. These precise chambers are cut to SAAMI (Sporting Arms and Ammunition Manufacturers Institute) specifications, not Mil-Spec (military) dimensions. As such they are slightly smaller and tighter than Mil-Spec chambers. While many perpetuate the myth that it is perfectly safe to fire the visibly similar 5.56mm rounds in .223 Remington chambered weapons, this is definitely not the case.