Dear Joe - believe I stated that the 5.56mm may indeed be loaded to higher pressures than the 223 commercial stuff.

I just was interested in finding an authentic source for this information, such as some sort of data from a reliable testing lab or manufacturer. In addition, I would be interested in seeing actual pressure test results for both types of loads.

I once rechambered a Remington M722 from .222 to .223. It shot commercial as well as GI ball ammo with no problems. But the GI stuff was not too accurate due to the 1/14" twistand the long, pointed 55-grain boattailbulllet. When I switched to55-grainflatbase bullets it shot quite well. However, I saw no evidence of excessive pressures with GI ball ammo.

Per Wikipedia:
" .223 Remington


.223 Rem next to .30-30 (center) and .308 (right)

Type
Rifle/varmint

Riflingtwist 1/12" (typical)

Primertype small rifle

The .223 Remington is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO military cartridge. It is loaded with a 0.224-inch diameter, jacketed bullet, with weights ranging from 40 up to 90 grains, though the most common loadings by far are 55 grains.
The primary difference between .223 Remington and 5.56x45mm is that .223 is loaded to lower pressures and velocities compared to 5.56 mm. .223 Remington ammunition can be safely fired in a 5.56 mm chambered gun, but the reverse can be an unsafe combination. The additional pressure created by 5.56 mm ammo will frequently cause over-pressure problems such as difficult extraction, flowing brass, or popped primers, but in extreme cases, could damage or destroy the rifle. Chambers cut to .223 Remington specifications have a shorter leade (throat) area as well as slightly shorter headspace dimensions compared to 5.56 mm "military" chamber specs, which contributes to the pressure issues."That means that advertised pressure of 58,000 psi (400 MPa) for 5.56 mm NATO, is around 78,000 psi (540 MPa) tested in .223 Rem test barrels. The 5.56 mm chambering, known as a NATO or mil-spec chambers, have a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber (Rock River Arms)[1] or the Armalite chamber, which are designed to handle both 5.56 mm and .223 equally well."

It seems to me that it is NOT ONLY just thefact that the GI stuff is loaded to higher pressures, but that civilian .223 barrel leades don't swage down the bullets as gradually as they enter the bore. This could cause any trendency for GI ammo to develop higher pressures even worse.

SAAMI max pressure for the .223 seems to be 55,000 PSI, whereas GI stuff can be as much as 58K PSI when shot in proper barrels. This 3,000 PSI difference would not be enough to be dangerous, BY ITSELF! It is the fact that the chambers can be different that potentially causes the trouble. Unless the civilian chambering specs result in a great improvement in accuracy over the GI type, I think it is really ridiculous to use two different sets of chamber specifications for what is essentially the same cartridge!!