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There are hundreds of articles written about the difference between .223 and 5.56 ammunition. The .223 measurement is an SAE measurement, while the 5.56 is a metric measurement. However, if you use a converter to convert the measurements, they do not calculate correctly. There are a couple of reasons the calculations do not coming out correctly: 1) One is the measurement of the lands from one side of the opening to the other, while the other is the measurement of the diameter of the bullet. 2) There are different methods and numbers used to calculate the conversion between SAE and metric measurements. Other considerations: 1. The thickness of the brass - causing the ratio of air to powder to be different, which will affect the pressure. 2. The type and seating of the projectile - causing the ratio of air to powder to be different, which will affect the pressure. 3. The primers used to ignite the powder - harder or softer, faster or slower. 4. The type of powder - slow burning or fast burning. Obviously, the above “other considerations” are going to affect the pressure in the chamber of a .223/5.56 firearm much more than whether the brass is .223 or 5.56. That’s not to say there is no affect from what the structure of the brass is, it’s just that it is not as important as the “other considerations” that can be varied much easier and accurately than the size, weight, thickness, composition, and stiffness of the brass itself As a loader or a reloader, the most important is consistency. If you can chamber the manufactured ammunition and depend on it to do the same thing every time you pull the trigger, then that’s what is important. In conclusion, the .223/5.56 argument is almost a moot point. There is not much of importance or substance to argue about. All modern firearms these days will accept, chamber, and fire either the .223 or the 5.56 with no issues. Issues that do arise will most likely be due to one of the “other considerations” listed above, firearm malfunction, faulty gun manufacturing, or user error. We have manufactured millions of rounds of .223/5.56 ammunition. When testing ammunition, we usually use a Mini 14 or an AK, which are reliable and will chamber the most variations of .223/5.56 ammunition. It’s not that our ammunition won’t work in an AR-15’s, we just don’t want reliability issues when testing. That’s what you should strive for when sending a bullet down range - Consistency!