As far as military and civilian firearms are concerned, people talk about little else other than 5.56 NATO versus .223 Remington ammunition. It might seem like the actual differences between the two cartridges are pretty darn small, but those differences have huge impacts on performance, safety, and overall weapon function.
The key difference lies in pressure. The 5.56 NATO runs at 58,000 psi, while the .223 Remington is loaded to 55,000 psi. That tiny difference can produce some major differences in performance and safety.
Another very important difference is the length of the chamber throat. The 5.56 NATO chamber has a throat that’s lengthened .125″ over a .223 Remington. That extra length allows, very roughly, one additional grain of powder into the 5.56 NATO cartridge, thus making it hot.
These differences, however, bring with them great risks if these two cartridges are used interchangeably. A 5.56 NATO, fired in a rifle that is chambered for a .223 Remington, may be capable of attaining as much as 65,000 psi or higher, its working pressure, instead of just the standard working pressure of 55,000 psi for a .223 Remington. This can create several dangerous scenarios, such as backing out of primers or even injury to the operator and the rifle.
By contrast, 5.56 NATO rifles chambered with the .223 Remington cartridge will have inferior performance. The lower pressure and powder charge of the .223 Remington, combined with those of the 5.56 NATO, may not be able to generate velocity and performance, especially in rifles with barrels shorter than 14.5 inches. This could mean that the rifle cycles poorly, although, with other rifles that have a longer barrel length, it should work with .223 Remington ammunition.
Whereas the outside dimensions of these cartridges are almost similar, experts in the field, for instance, those of AR15barrels.com, go ahead to assert that it is the chamber dimensions and the throat lengths that take center stage in performance and safety. Over time and with throat erosion, these dimensions may be affected—a further complexity in the matter.
However, in practical terms, a velocity drop of say 100 fps at the muzzle and minor accuracy degradation when firing .223 Remington ammunition in a 5.56 NATO-chambered rifle are largely irrelevant for self-defense out to 100 yards; these distinctions increase in precision shooting.
The bottom line is, that while 5.56 NATO ammunition can be unsafe in a .223 Remington chamber due to potential spikes in pressure, .223 Remington is almost always safe in a 5.56 NATO chamber. How well .223 Remington ammunition performs in semi-automatic rifles will depend more on the firearm’s setup rather than on the ammunition itself.
These subtleties would be realized only if the user understands them, which is something very essential for any stakeholder using these cartridges, be it in the military, law enforcement, or civilian use. The differences, though minute, are immense in terms of safety and performance.