Winchester Small Rifle 5.56mm NATO-Spec Military Primers #41

Winchester small rifle

As with most factors related to 5.56 and 223, actual case capacity will vary. In summary: the exterior dimensions of 223 Remington and 5.56 NATO ammunition are essentially the same. 223 Remington and 5.56 NATO cases are essentially the same, perhaps with 1% difference in case capacity between the two.

Where much of the confusion comes when trying to understand the differences between 223 Remington and 5.56 NATO ammunition is when reading the published maximum allowed pressure for each cartridge. You may see data published that looks like this:

…and this data would make it appear that 5.56 NATO ammunition is certified “to a higher pressure” than 223 Remington ammunition. That is actually not the case! It’s only when we observe the US Military’s updated pressure testing and rating data when we see a parallel:

The conclusion: 223 Remington and 5.56 NATO have essentially the same maximum working pressure rating. So what about the “interoperability issues” between 223 Remington and 5.56 NATO then? These are real, some of the time. Let’s first compare the chambers between .223 Remington and 5.56 NATO.

There is some information that’s easy to find on the internet and some that must be gathered from reputable online sources and industry players (large companies, smaller reamer grinding shops, and everything in between). I’ll note here that chamber dimensions seem to be somewhat “open to interpretation” by reamer manufacturers and firearms OEMs. So I’m not going to state anything as universal fact here: these are numbers that I feel best to represent the differences between 223 Remington spec chambers and 5.56 NATO military spec chambers. In short, reamers and chambers will vary, and the most important thing is how firearms are certified and tested. As seen in the video, here are the differences in chamber dimensions between 223 Remington and 5.56 NATO