5.56 NATO cartridge dimensions.
December 06, 2024
I have a question for Lane Pearce, and it stems from a former coworker who was strongly opinionated that .223 Remington and 5.56 NATO are not interchangeable due to headspace issues. Can Lane shed any light on that? — Greg Bell, via email
Although I’ve answered this question before, I conducted some fresh research in order to provide a thorough answer. In addition, I came across a treatise recently prepared by my friend Richard Mann, so I want to acknowledge his contribution right here and now. For all practical purposes, the two rounds are dimensionally identical, although there may be some minimal differences that would most likely be within the tolerance limits of either round. Because it’s a commercial cartridge, the .223 Remington’s physical and ballistic criteria are controlled by SAAMI specifications. But the 5.56 NATO is a military cartridge used by the United States and several international armed forces, so it is not listed in SAAMI records. That said, some sources report that the 5.56 NATO case walls are slightly thicker, so the .223 Remington has a bit more case capacity. I’ve compared the weights of commercial and military brass, and the delta was in the noise level of normal lot-to-lot variation. Therefore, I do not reduce charges when reloading military brass versus commercial brass. The only extra case-prep step required is a one-time removal of the primer crimp in the military brass. Please note, you’ll likely run into some commercial headstamp ammo with the military primer pocket crimp. Pay attention! According to Mann’s report, the .223 Remington was introduced around 1962. The 5.56 NATO round wasn’t official until about 1970 and was not adopted by our international partners until 1979. As many readers know, the AR-15/M16 was deployed to our troops in Vietnam from the mid-1960s to the early 1970s, so the 5.56 round’s specifications were still in transition. Today, the two significant differences are 1) allowable chamber pressure limits and 2) actual physical differences in the chamber leade dimensions. The .223 Remington’s maximum average pressure (MAP) is specified at 55,000 psi by SAAMI. The 5.56 NATO is significantly greater at 62,000 psi. In addition, the chamber dimensions for the .223 Remington have a substantially shorter leade (or freebore) and a sharper angle at the origin of the rifling. The original .223 Remington ammo was loaded to a MAP of 55,000 psi with 55-grain JSP bullets as a varmint cartridge, so commercial rifles were/are chambered to allow minimal bullet jump to achieve optimal accuracy. The original 5.56 ammo was topped with 55-grain FMJ bullets at who knows what pressure. While the M16 and 5.56 were involved in real-world field testing in the jungles of Southeast Asia, the cartridge and rifle were significantly altered based on unexpected numbers of casualties.
.223 Remington cartridge dimensions. The rifles’ chambers and bores were chrome lined, and the untested double-base spherical powder that left residual calcium carbonate in the chamber was replaced by the original, single-base extruded propellant that did not foul the bore. That eliminated the problem with the extractor ripping the rim off, causing a fired case to stick in the chamber, and render the rifle inoperable. And, of course, over the years the military revised the 5.56 specs repeatedly to “enhance” battlefield performance. Bullet weight increased to 62 grains, then 77 grains, etc., etc. Heavier .22-caliber bullets are longer, hence the need for an extended chamber throat and shallower leade angle into the rifling to avoid excessive chamber pressures. Mann stated that firing a current-production, heavy-bullet 5.56 NATO round in a commercial .223 Remington-chambered rifle could result in chamber pressures reaching/exceeding 70,000 psi. (SAAMI specs for .223 Remington proof pressure is 78,500 psi.) He acknowledges you might get by with shooting one round or maybe even 10 or more, but the potential for a catastrophic event is lurking every time you press the trigger on a NATO round in a .223 Remington chamber. The reverse scenario (firing .223 Remington ammo in a 5.56-chambered rifle) is totally acceptable. The only downside is accuracy will likely suffer because the lightweight/shorter bullet is not adequately supported in the longer 5.56 throat. Someone developed the .223 Wylde chamber concept years ago to help alleviate this incompatibility. It’s not a different cartridge. It is a chamber reamer that’s sort of a dimensional hybrid of the two original chambers. The good news is rifles so chambered seem to work well with either type of ammo. To be safe, do not fire 5.56 ammo in rifles marked .223 Remington.
