The .223 Remington and 5.56 mm NATO are similar, but differences do exist.
With the popularity of AR-15 rifles surging over the last 10 years, interest and demand have increased for the AR-15’s most popular chambering, the .223 Remington. The 5.56 mm NATO cartridge is a variant of the .223 Remington, but the two cartridges are not the same. There are differences that a shooter or handloader must be aware of.
The .223 Remington and 5.56 mm Projectiles
The .223 Remington cartridge was developed in 1964. It fires a .224-inch diameter bullet, with bullet weights ranging from as low as 40 grains for varmints to as high as 90 grains for big game hunting applications. The 5.56 mm NATO cartridge has the same exterior dimensions as the .223 Remington. It also fires a .224-inch projectile in a wide variety of bullet weights, but, being a military cartridge, it is produced with bullets developed specifically for battle applications, such as tracers.
While the .223 Remington and 5.56 mm NATO share similar exterior dimensions, the chambers from which they are fired are different. The distance from the case mouth to where the rifling starts is called freebore or leade. The leade for a 5.56 NATO chamber is approximately twice as long as for a .223 Remington. The 5.56mm NATO chamber leade is .162 inch, while the .223 Remington’s leade is .085 inch.
The 5.56 mm NATO’s brass casing has greater wall-thickness than the casing for a .223 Remington. This is because the 5.56 mm NATO develops higher chamber pressure. The 5.56 mm NATO develops 62,000 psi, while the .223 Remington produces at 55,000 psi.
Due to the differences in leade and pressure, the cartridges are not interchangeable. The .223 Remington can be safely fired in a rifle with a 5.56 mm NATO-spec chamber, but it is extremely dangerous to fire a 5.56 mm NATO round in a .223 Remington chamber. The operating pressure alone makes this a dangerous practice. Also, pressure can dramatically rise beyond safe levels because the 5.56 mm NATO would not have enough leade in a .223 Remington chamber.