This is probably a dead issue for some
but I want to pass on the information that I know since I have been in the military for 22 years now and my AR and M4 have been my companion for 20 years (the first 2 was a M14)
. I have made quite a few AR's and love them dearly. I have seen the question partially answered and wanted to get this info out for you. The difference between the two depends on the rifle! If you have a rifle chambered for .223 then the lead (The Lead is the measurement from which the rifling starts on the barrel from the breach) on that rifle will be .085" if you have one chambered in 5.56 NATO then your lead is .162". Therefore if you use a 5.56 round in a .223 rifle the lead is so close that the projectile will be forced out right at the rifling of the barrel and create high pressures that either 1 your casing will fail or 2 your bolt could fail or the worst 3 your upper receiver will fail and in my opinion all are bad juju!
If you are chambered for 5.56 then all is good. This is why you lose accuracy and velocity when using the .223 in your 5.56 chambered AR. that little gap gives just a bit of wobble and that takes away your twist and pressure when using 223. That is why when one person put it you can use the 223 for targets but leave the 5.56 for zombies. Another point is that the casing is thicker in 5.56 than a .223 for pressure reasons. Another thing that was brought up is the lands and twist, I prefer that if you are chambered for 5.56 use the 1:7 this gives optimal velocity for your average 55 grain if you have a .223 a 1:9 is optimal for the same reason velocity. Even though I have no evidence to that as of yet but that is my opinion. I will try that in my next build. Thank you for your time.