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.
Cartridge
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
Chamber
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;
- Freebore diameter
- Freebore length
- Leade (or throat)
- 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]
Interchangability
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.
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 - http://www.olyarms.com/index.php?option=com_content&view=article&id=26:how-is-my-oly-arms-barrel-chambered&catid=7:general-questions&Itemid=2
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.
