In the convoluted world of ammunition, the .223 Remington and 5.56 NATO cartridges hold a unique place.

Yet, these two rounds are a study in contrasts, a nuanced distinction that can spell the difference between optimal use and potential peril.

The disparities between these two cartridges may seem slight, but they wield substantial influence over performance, safety, and firearm operation.

Firstly, the 5.56 NATO cartridge operates at a higher pressure level, approximately 58,000 psi, compared to the approximately 55,000 psi of the 223 Remington.

The second and most critical discrepancy lies in the design of the chambers. A 5.56 NATO chamber features a throat that is .125” longer than that of a 223 Remington chamber.

This additional length permits the loading of approximately one extra grain of powder into a 5.56 NATO cartridge, contributing to its superior performance when compared to its 223 Remington counterpart.

The primary concern arises when a 5.56 NATO cartridge is fired in a rifle chambered for 223 Rem.

The longer throat of the NATO chamber causes the 223 chambered weapon to operate at pressures of approximately 65,000 psi or higher—10,000 psi beyond the 223’s standard operating pressure of 55,000 psi.

This situation is unsafe and can result in primer back out or, worse, cause harm to the operator or damage the rifle.

Conversely, firing a 223 Rem cartridge in a 5.56 NATO chambered rifle presents its own set of issues.

The difference in throat length between the chambers can lead to suboptimal performance of the 223 Rem cartridge in a 5.56 NATO chambered weapon.

This stems from the failure to achieve the 55,000 psi pressure typical of the 223 Rem cartridge when fired in a 5.56 NATO chamber, consequently affecting velocity and overall performance.

Problems arise particularly when this combination is fired from a 5.56 NATO chambered rifle with a barrel length of 14.5” or shorter.

The reduced powder charge of the 223 round, combined with the pressure drop experienced when fired in a 5.56 NATO chamber, can lead to improper cycling of the rifle.

However, NATO chambered rifles with barrels longer than 14.5” should function adequately when firing 223 Rem ammunition.

Relevant articles:
– What is the difference between 5.56 NATO and 223 Rem ammunition?, Hornady Law Enforcement
– What’s the Difference Between .223 Remington and 5.56 NATO?, themeateater.com
– .223 vs 5.56: Comprehensive Caliber Comparison by Ammo.com, Ammo.com
– 5.56 vs .223: What’s the Real Difference Between These Popular Cartridges?, Outdoor Life