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04 February 2013

223 Remington vs. 5.56 mm – The Definitive Article

223 Remington vs. 5.56 mm – The Definitive Article

There is probably more mystery and misinformation surrounding the 5.56 mm and .223 Remington calibers than any other. This article will attempt to clear up some of the misinformation.


The .223 was derived from the .222 Special. In early 1962, Remington submitted specifications to SAAMI for the .223 Remington. They then introduced the cartridge commercially in January 1964. Shortly thereafter, the same cartridge was adopted by the U.S. military as the 5.56 x 45mm.
In 1980, the 5.56 x 45mm was specified as the 5.56 NATO (no “x 45mm”)

While the .223 was “locked in” to the SAAMI specifications, the 5.56 continued to evolve, resulting in the changes we see today.

It is important to separate the issues between cartridge differences and chamber differences.


The 5.56 and .223 case dimensions are practically identical. The .223 dimensions as specified by SAAMI are given in inches. 5.56 dimensions are in millimeters.[*]

Some mil-spec cases have slightly thicker walls in the head area to accommodate the higher pressures of military loads. This case wall thickness is evident in the official case capacities;

.223 Remington - 28.8 grains
5.56×45mm - 1.85 ml (28.5 grains)

In reality, case capacities vary considerably by manufacturer and even lot:

Lapua – 29.2 grains
Federal Lake City – 30.4 grains


There are several different chamber profiles for both the .223 and the 5.56. In addition to the SAAMI and NATO profiles, there are the Wylde, Derrick, AMU and many others. Since the SAAMI and NATO profiles generally represent the dimensional extremes with all others being modifications of these two, we will only consider SAAMI (Sporting Arms and Ammunition Manufacturers Institute) and NATO mil-spec[†] chambers.
There are 4 primary differences between the 5.56 NATO and .223 Remington chambers;

  1. Freebore diameter
  2. Freebore length
  3. Leade (or throat)
  4. Leade angle

Simply put, freebore is the cylindrical area directly in front of the cartridge mouth where there is NO rifling. It is the same or slightly larger diameter than the projectile. Leade or “throat” is the transition between the freebore area and full rifling. Some people use the term throat or leade to include the freebore area also.

Chamber Markings

To further complicate matters, manufacturers sometimes fail to mark their chambers or mark them with the wrong identification. Below are listed details by manufacturer.

ArmaLite          Modified SAAMI (Wylde) in its stainless steel match barrels. 
NATO chamber in all moly (phosphated) and chrome-lined barrels[1]

Bushmaster      Chambered in 5.56mm.[2]

Colt                NATO[3]

Olympic           Button rifled barrels are chambered in 5.56 NATO
SUM (Stainless Ultra Match) barrels are chambered with Clymer® .223 Remington reamers cut to minimum SAAMI specs.[4]

Rock River       Modified SAAMI (Wylde)

Ruger              Mini-14 and Ruger No. 1 rifles manufactured since 1986 – NATO[5]


Armalite’s TECHNICAL NOTE 45;

Millions of rounds of NATO ammunition have been fired safely in Eagle Arms' and ArmaLite’s  SAAMI chambers over the past 15 years. Occasionally a non-standard round (of generally imported) ammunition will fit too tightly in the leade, and resistance to early bullet movement can cause elevated chamber pressures. These pressures are revealed by overly flattened or powder stains that reveal gasses leaking around the primer.
The first few rounds of ALL ammunition, from whatever source or lot, should be checked for pressure and other signs of defect before firing large quantities. If you have a problem, you can generally bet that the ammunition meets neither SAAMI nor NATO specifications.

From Federal Cartridge;

The 5.56mm military cartridge fired in a 223 Rem. chamber is considered by SAAMI (Small [sic] Arms and Ammunition Manufacturers Institute) to be an unsafe ammunition combination and is listed in the “Unsafe Arms and Ammunition Combinations” Section of the SAAMI Technical Correspondent’s Handbook. It clearly states; “In firearms chambered for 223 Rem – do not use 5.56 Military cartridges.”
Federal Cartridge is a member of SAAMI and supports this position.

My Opinion;

There are literally dozens of different chambers, and hundreds, if not thousands of cartridge loads. The chances of actually having a combination that is dangerous is very slim. Improper weapon handling and maintenance and defective ammunition pose a much higher threat to safety than 5.56 ammo in a .223 rifle. Hopefully with the information in this article the reader can make their own informed decisions.

[1] Armalite Technical Note 74, Rev 1, 04 April 2011
[2] Bushmaster 2006 Catalog, V1, Pg 6
[3] Rifle magazine January/February, 1988 pages 36, 37, 70 and 71. Written by John Schaefer
[4] Olympic Arms website, support section - Accessed 29 Jan 13
[5] Rifle magazine January/February, 1988 pages 36, 37, 70 and 71. Written by John Schaefer

[*] SAAMI gives a specification for the r2 dimension, the Radius of junction between junction cone and collar. Mil-specs do not.

[†] There is no NATO standard chamber. NATO STANAG (Standardization Agreement) 4172 addresses only the 5.56mm cartridge. Individual countries’ military specifications address chamber dimensions.