OK, even DPMS could not explain it right. Here is some information I gathered from various sources, including a gent who may pop in here from time to time,
the 45B class and the fine folks at Crane. Funny how the real experts all say the same thing about the 5.56 and .223 differences.
Can you fire 5.56 in a .223? Well, in short yeah you can. Are there issues with it, well maybe so..... Will it blow your gun apart and send pieces of the gun thru your brain housing group, probably not. But lets take a look at what the issues are for reals.
First of all there are three major types of AR chambers. The .223 SAMMI, the .223 Wylde and the 5.56 NATO. It's all so crazy and convoluted that we will skip right to the point of it all and look at it in layman's terms.
The real issues are the case neck, free bore length, throat angle and headspace. Now what do they each mean?
Case neck- Thats where the bullet sits in the case.
Free bore length- Thats the length between cases end and where the lands and groove start. This is where the bullet is actually free in the bore when it leaves the case. This affects the pressure of the round as the lands and grooves cause pressure to build behind the bullet.
Throat angle- This is the angle of slope where the free bore ends and transitions into the lands and grooves.
Headspace- This is the depth of the chamber
Now the headspace on all chambers is the same at 1.4636, so that is a non issue. The throat angle is significantly different though, 3 degrees for the SAMMI and 1.25 degrees for the other two. How does this affect you? It makes the pressure in the SAMMI start quicker as the pressure against the round is applied more abruptly.
Free bore length is a big issue in the three. The free bore for the SAMMI is a mere .025, the Wylde is .062 and the 5.56 NATO is .057. What does that mean? It means that the SAMMI builds pressure a lot faster than the Wylde or NATO chambers. Add this to the fact that NATO rounds are already a few hundred feet per second faster and as much as 5,000 PSI higher in pressure and you have an issue building rapidly. As is the pressure.
Does this mean that your gun is going to explode by using NATO ammo in your .223, probably not. They are massively over engineered and generally pretty safe for these kinds of things. I personally have fired thousands and thousands of NATO rounds in a .223 chamber. They are designed to take pressure spikes and if God blows the weapon up in your face, he is pissed at you and you would have gotten screwed some other way if you had not been shooting at the time. But it is NOT recommended all the same.
Thanks again to all the sources that helped me compile this info.