You heard wrong. I strongly recommend to NOT shoot 5.56 in a .223 chambered firearm.
5.56 chambered firearm: can shoot either 5.56 or .223
.223 chambered firearm: ONLY .223.
Same goes with 7.62x51/.308.
Only shoot .308 in a .308 chambered firearm.
A 7.62x51 chamber can shoot both.
And it's not that they will function, but its that they both can SAFELY.
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