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It can fire both .223 and 5.56 since the barrel is marked 5.56 (normally higher pressures than .223). Some do not recommend firing 5.56 ammo in a .223 chambered barrel, but it’s okay to fire .223 in a 5.56 chambered barrel.
Couple questions: 1) M4 Carbine or just Carbine? 2) letter stamped in the barrel up near the chamber? 3) date under gas tube at the gas block?