The .223 Remington cartridge was introduced by Remington Arms in 1957. It was based on the .222 Remington cartridge, but with slight modifications to the case dimensions and bullet design. The .223 Remington quickly gained popularity among civilian shooters due to its accuracy, low recoil, and versatility for various shooting applications such as target shooting and varmint hunting.

In the 1960s, the United States military sought a new intermediate cartridge to replace the .308 Winchester (7.62x51mm) in certain military rifles. The result was the development of the 5.56x45mm NATO cartridge, which is commonly referred to as the 5.56mm NATO.

The 5.56mm NATO cartridge was designed to meet specific military requirements, including enhanced terminal performance, effective range, and reliability in various combat conditions. It featured a longer, throated chamber known as the NATO chamber, which allowed for the use of longer and heavier bullets while maintaining compatibility with the shorter .223 Remington ammunition.

One key difference between the two cartridges is the chamber dimensions. The .223 Remington has a slightly shorter leade (the area in front of the chamber where the bullet makes its initial contact with the rifling) compared to the 5.56mm NATO. The longer leade in the 5.56mm NATO chamber contributes to its ability to handle longer bullets and potentially higher chamber pressures.

Another difference lies in the pressure levels. The 5.56mm NATO is loaded to higher pressures than the .223 Remington, primarily to ensure reliable functioning in military rifles under adverse conditions. The higher pressures contribute to the 5.56mm NATO's increased muzzle velocity and terminal ballistics.

It's worth noting that while it is generally safe to fire .223 Remington ammunition in a firearm chambered for 5.56mm NATO, the reverse is not always true. The higher pressures of 5.56mm NATO ammunition may exceed the design limits of some firearms specifically chambered for .223 Remington. Therefore, it's important to consult the firearm manufacturer's recommendations and markings to ensure safe ammunition usage.

In summary, the .223 Remington and 5.56mm NATO cartridges share similarities but have distinct differences in chamber dimensions and pressure levels. The 5.56mm NATO was developed for military use, featuring a longer chamber throat and higher pressures, while the .223 Remington is a civilian cartridge known for its accuracy and versatility. Understanding these differences is crucial for selecting the appropriate ammunition for a specific firearm and ensuring safe and reliable performance.