When the .223 and the M16 first came out they "BOTH" had the same 1 in 12 twist, with the advent of longer and heavier bullets to "improve" the M16 round the twist rate was changed in the M16 to a 1 in 7 twist. To accomplish this the throat was lengthened on the M16 to accommodate the longer bullets.
When the throating became different between the 1 in 12 twist .223 and the 1 in 7 M16 rifle is when the "change" took place and higher pressure spikes could happen in commercial .223 rifles with the shorter throat.
There is "NO" difference in chamber pressures between the .223 and the 5.56 NATO round. The confusion comes from NATO EPVAT pressure testing methods where the pressure is measured at the neck of the case verses mid case for SAAMI pressure testing.
My Stevens 200 .223 has a 1 in 9 twist and is long throated and can safely fire both commercial and military ammunition.
Both the SAAMI and the European CIP conceder the .223 and 5.56 NATO to be the same cartridge, with the exception of twist rates and the corresponding throating differences.
The civilian and military 30-06 are for all practical differences were the same, the .308 and 7.62 were the same and the .223 and 5.56 are the same. The only differences happened when the military changed the twist and bullet weight for the M16 rifle which effected the older commercial .223 with the 1 in 12 twist and the corresponding SAAMI warning about ammo interchangeability.
Below, two five gallon buckets of .223/5.56 cartridge cases in my basement segregated by make and type. If you would do a little research yourselves instead of relying on rumors and myths you would already know what you have just been told.