Well, we all know how this is as a source, but it's a good summary, I've found comments to this effect elsewhere too...
http://en.wikipedia.org/wiki/5.56
[edit] 5.56mm NATO versus .223 Remington
While the 5.56mm and .223 cartridges are very similar, they are not identical. Military cases are generally made from thicker brass than commercial cases; this reduces the powder capacity (an important consideration for handloaders[1]), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000 psi (140 MPa) difference in pressure measurements. That means that advertised pressure of 58,000 psi (400 MPa) for 5.56mm NATO, is around 78,000 psi (540 MPa) tested in .223 Rem test barrels. The 5.56mm 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)[2] or the Armalite chamber, which are designed to handle both 5.56mm and .223 equally well.
Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the longer leade.[3] Using 5.56 mil-spec cartridges (such as the M855) in a .223-chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice.[4] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56mm, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56mm ammunition.[5]
Then from DPMS's site:
All DPMS rifles chambered in .223 Remington (stamped on the base of the barrel) are designed for use with .223 ammunition only. Failure to use .223 ammunition may cause unsafe conditions and damage your rifle.
and
All DPMS rifles chambered in 5.56x45mm (stamped on base of barrel) are able to use both 5.56x45mm and .223 Remington ammunition.
Kinda answered my own question, been poking around here and there all morning reading stuff.