Well.... I wouldnt take this as definate knowledge but I do believe mill spec rounds are a little higher pressure as they are designed for large machine guns. The primers are definatly harder as well. I have shot mill spec stuff but it was through a bull barrelled rifle and had no overpressure problems. I had several failures to fire due to the hard primers but never a ft fire with commercial ammo.
George Orwell: "That rifle on the wall of the labourer's cottage or working class flat is the symbol of democracy. It is our job to see that it stays there."
Check http://www.303british.com/. Both rounds have the same external dimensions. 308 rounds may have a bit higher pressure, but the main problem is that 308 brass is thinner and mil 7.62 chambers are a bit looser to allow feeding under adverse conditions. Theoretically one should check the headspace in military guns before shooting high pressure commercial ammo to avoid case streching (or rupture!) problems. I never had a problem with this.
Check out THE U.S. .30 CALIBER GAS OPERATED SERVICE MANUAL VOLUMES I & II by Jerry Kuhnhausen; pages 17,21,23,36,43,&44. Headspace requirements are different. The 7.62mm Nato and the .308 Winchester are not the same dementionally.
If it (a FAL, CETME, HK, etc...) is headspaced for 7.62 NATO it is supposed to eat 147 grain ball ammo. However, the same rifle can USUALLY accept 168 grain .308 civilian ammo.
BUT... BUT... BUT... there are significant differences in some of the newer hot .308 loads that are to be used in deer rifles etc. Do not use these rounds in military rifles due to adverse headspace tolerances and vice versa (don't use 7.62 NATO 147 grain in rifles headspaced for hot big game rifle loads which expect high quality brass).
In the two extremes... headspace and brass strength can cause problems for each other.
OK, I read the web article and that made sense... but now I have to ask the question: do 5.56mm and .223 Rem exhibit the same types of differences as well? I had thought these were suppose to be "the same" in the same way that 7.62 & .308 were "the same"...
"He who goes unarmed in paradise had better be sure that that is where he is."
-- James Thurber
They are not the same!!
Please go to: http://www.thefiringline.com/forums/showthread.php?s=&threadid=15031&highlight=223+vs+5.56
"ACCORDING TO SAAMI (THE PEOPLE WHO SHOULD KNOW) THERE IS NO DIMENSIONAL DIFFERENCE IN .223 AND 5.56mm CARTRIDGES. THE DIFFERENCE IS IN PRESSURE LEVELS OF THE LOADED CARTRIDGES.
.223 is lower pressure (about 52,000 CUP) and 5.56mm is designated as higher pressure. SAAMI has long stood by their recommendation that 5.56 not be used in rifles labeled as .223 because of the risk of overpressure. It may never happen in your gun, but it's like using 38+P in a revolver designated as 38 only.
There are also differences in chambers. THIS IS ALSO NOT A SAAMI REGULATED DIMENSION. Military rifles and some others use a SLIGHTLY larger chamber to facilitate feeding/extraction, especially under sustained fire and in adverse conditions. Remember, the first and most important thing about a combat arm is RELIABILITY, thus, the larger chamber.
It's really usually only a matter of running a finishing reamer through the chamber after the rough reamer a few more times than usual to get a 5.56 chamber instead of a .223 chamber. THEY ARE THE SAME TOOLS. HOWEVER, some manufacturers use finishing reamers that are slightly larger, thus facilitating faster prodiction and lower tool wear."
Some manufacturers (like Ruger) used to chamber their guns for 5.56 to avoid problems, I don't know now.