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 convolutted that we will skip right to the point of it all and look at it in laymans terms.
The real issues are the case neck, freebore length, throat angle and headspace. Now what do they each mean?
Case neck- Thats where the bullet sits in the case.
Freebore 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 freebore 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 pressure is appled more abruptly.
Freebore length is a big issue in the three. The freebore for the SAMMI is a mear .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 oressure 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. They are designed to take pressure spikes, however I DO NOT RECOMMEND IT. It simply is not safe. You WILL probably start to see primers popped out of the cases or swollen.
So what do we do about it all? Well, we try to order all of our AR's in the 5.56 chambers. That allows you to shoot whatever you want to in the gun. The next thing you can do is shoot .223 ammo in your guns. Milsurp is not as cheap as it used to be, so there is not much advantage to go that route anymore. But if you do find a deal having a 5.56 chamber is going to make life more comfortable for you. But if you dont have a 5.56 chamber, just dont shoot milsurp ammo.