I looked into this. There are extensive threads on FTF, which I have contributed to, in the ammo sub-forum I believe, and elsewhere concerning this.
The gist of it is there are small differences in the neck/throat area of the cartridges and corresponding chambers that result in higher pressures in a military 5.56. It was adopted from the civilian 223, but I believe it was changed to provide more energy in order to reliably cycle a dirty automatic weapon under adverse conditions, which a bolt action .223 didn't need to do.
You can shoot a 5.56 out of a .223 firearm but it’s supposed to fatigue the weapon over time and may prove dangerous. I don’t know how many rounds that would take. There are others who load ‘hot’ 223s and use them without issue, so I don’t know whether this is just a theory or a proven fact.
You can shoot a .223 out of a 5.56 without any problem, but may suffer a slight accuracy loss. That said, I would think the difference would be way smaller that one’s own inconsistency in marksmanship.
Rock River Arms’ AR15s, and some others, use a ‘Wylde’ (sp?) Chamber’ which is designed to shoot either without any difference in fatigue or accuracy.
Mossberg makes a bolt action that’s chambered for 5.56. I don’t know of any others. It seems to be rare relatively speaking. You would think there would be more given the flexibility of ammunition a ‘dual' .223/5.56 chambered weapon could use.
And if I wrong I’m sure someone here will let us know straight away.
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