223 vs 5.56, these rounds are considered interchangeable by most shooters, but the truth is they’re not identical. Actually, there are wide variety of rounds that can be called either .223 or 5.56, but this discussion focuses on the two most popular cartridges, the .223 Remington and 5.56x45mm NATO.
The .223 Remington, also known as the .223 Rem, was first developed in 1962 as a civilian sporting cartridge built for the Remington 760 rifle. Two years later the .223 Rem was adopted by the United States Army after the ArmaLite AR-15 rifle was first deployed in combat as the M16A1. The M16 would eventually replace the M14 as the U.S. Military’s standard service weapon in 1969. The U.S. Army’s official name for the .223 Rem is the 5.56x45mm Ball, M193.
In 1954, the 7.62x51mm NATO cartridge became the first standard NATO rifle cartridge. In 1977, NATO agreed to choose a second, smaller cartridge as a replacement for the 7.62x51mm. A modified version of the .223 Rem was created by FN Herstal as the 5.56x45mm NATO, also known as the M855 by the United States Military.
The most notable difference between the 223 Rem and the 5.56 NATO are pressure and chamber dimensions.
SAAMI specifications for the .223 Rem were first submitted by Remington in 1964 using DuPont IMR Powder. The original pressure was listed as 52,000 psi, but this value was increased to the current pressure of 55,000 psi after IMR powder was replaced with Olin Ball powder.
Official NATO proofing guidelines require every cartridge to be proofed at 125% of max pressure before it can be certified for service issue.
The 5.56 x45mm chamber is larger than the chamber designed for .223 Rem.
According to the .223 Rem and .223 AI Cartridge Guide by AccurateShooter.com, a 5.56x45mm NATO cartridge should not be used in a .223 chamber. Doing so places the bullet in contact with the rifling and creates much higher pressure than the .223 chamber was designed to handle.
This danger is reiterated by the Lyman AR Reloading Handbook Reloading Manual which states, “Our data was developed using a SAAMI specification .223 Remington chamber pressure/test barrel and then fired through a Colt AR for velocities. This data therefore would be safe in all three types of chambers. If a load is worked up to a maximum level in a 5.56 NATO or Wylde chamber, it may produce high pressure if fired in a .223 Remington chamber.”