Here's a recent article on the subject.
Differences Between
223 Rem. & 5.56 NATO
THE STORY ABOUT THE DEVELOPMENT AND EVOLUTION OF THE
5.56 NATO and 223 Remington cartridges is much too
long to relate here, but the differences between them can
be described with just a few sentences. From a strictly
physical perspective, the cartridge cases are essentially
identical. And, of course, the maximum cartridge overall
length is the same: 2.260 inches. Their ballistics and
chamber specs, however, diverge somewhat.
The 223 Rem. specifications are controlled by
S.A.A.M.I., a civilian organization comprising industry
members representing the major firearms and/or munitions
manufacturers. An international government body
representing the various member countries' military agencies
controls the 5.56 NATO requirements.
The .223 is typically loaded with bullets ranging in
weight between 35 and 60 grains. The twist rate of commercial
rifles chambered in 223 Rem. will typically be
from 1:9 to 1:12. With the 5.56 NATO, ammo is usually
loaded with 62- to 80-grain bullets, and 5.56 rifles typically
have faster 1 :7 to 1 :9 twist rates to assure the heavier
(i.e., longer) bullets are adequately stabilized.
The most significant differences between the two cartridges
are the chamber throat and leade dimensions.
The 223 chamber features a very short throat forward
of the neck and relatively steep leade angle. To accommodate
the heavier/longer bullet weights, the 5.56 NATO
chamber has a longer throat and shallower leade angle.
Maximum average pressure (MAP) limits differ also.
The SAAMI spec for the 223 is 52,000 CUP or 55,000 psi.
NATO limits the 5.56 to 62,000 psi; however, I believe the
actual position of the transducer relative to the case differs
between the civilian and military specs.
The bottom line is this: If you have a commercial bolt
gun chambered in 223 Rem., it may be hazardous to fire
5.56 NATO ammo loaded with bullets weighing more than
55 grains. A longer bullet, with the ogive jammed into the
short throat/leade, will surely cause chamber pressure
to exceed S.A.A.M.I. MAP specs.
Conversely, rifles chambered for 5.56 NATO will safely
accommodate commercial 223 ammo, but accuracy
may suffer when firing lighter-weight bullets because
the shorter bullet must "jump" across the longer throat
before the ogive engages the rifling, i.e., bullet axis could
be misaligned with the bore axis.
