One of the most annoying statements we hear in this industry is this:
"No I can't shoot 223, my rifle is 5.56. I am not going to damage my gun by shooting anything else".
If I had a nickel for every time I heard that, I'd probably be on some resort island enjoying retirement on a heap of nickels.
We use 2 main brass manufacturers for our 223 and 5.56 brass. One manufacturer explained to us that the only difference between the two is that they switch out the bunter that strikes the back of the casing with a headstamp. Anything measurable between the two casings is identical.
Our other manufacturer makes the casings the exact same as well, but they anneal the 5.56 a little longer to make it a bit harder to withstand the extra pressure in the 5.56 cartridge.
All in all, they're the same.
The projectiles loaded in both come out of the same box. There isn't a shred of difference in the 55gr Full Metal Jacket that gets loaded in one or the other.
The only difference here is the pressure within the cartridge itself. The 5.56 is loaded to a higher pressure than the 223. A rifle chambered in 5.56 will be absolutely unaffected in any way by the lesser pressure of a 223 round.
However, the pressure within the 5.56 isn't recommended for a rifle chambered for only 223. It might be just fine for a while, but repeated overpressure will likely cause problems later on.
Now go forth and correct all the misinformation! Together, we will right the wrong!