It's a myth that US made 5.56mm military cases are thicker than .223 commercial cases. i've weighed the cases of my accuracy re-loads for decades. Some British made 5.56mm cases are thick as are some Lapua and S&B cases. i had a couple S&B cases get into a batch of my Lake City re-loads that were max. The result was bad primer leak in my Remington 700.
Click on brass weights comparisons:
Lucky Gunner Labs tested 5.56mm ammo in .223 chambers, etc. One of the rifles with a "5.56mm chamber" gave higher pressures with military ball ammo than the same ammo gave when fired in a ".223 chamber". This bears out the claims of a good gunsmith and myself that .223/5.56mm chambers are all over the place in dimensions.
What Happens When You Fire 5.56 Ammo in a .223 Chamber?
I know most people are interested in this part – and yes, I really did fire 5.56 in a .223 chamber. I had a friend at the range with me at the time, and he stayed a good ways back when I did so. I was not concerned about what would happen, and as I expected, the results were not spectacular.
However, I was surprised about the results I saw. There was no dangerous spike in pressure from firing 5.56mm ammo in a .223 chamber. While pressures were elevated in comparison to one of the 5.56mm barrels, they were slightly below the other 5.56mm barrel.
The TW made 5.56mm military ammo is very accurate: Some of the stuff rivals expensive "match" ammo in accuracy. Using TW 67 M193 ball ammo, two of my bolt action rifles consistently make 5/8-3/4 five shot groups at 100 meters. That ammo has been stored in an undergound cellar since 1968.