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If it says 223 then it's 223. Is 5.56 also stamped on it? The manufacturer says it's .223/5.56 and the 556 in the model number also denotes it being 5.56. I'd call ATI to be sure. 800-290-0065

Fernando, This little pistol will handle 5.56 and 223 no problem

Fernando, since American Tactical maintains the Warranty, I would confirm firing 5.56 directly from ATI at 800-290-0065 or by email inquiry here : https://www.americantactical.us/contact.html

I would speak with some one at ATI about this issue until then I would only use .223 ammo 1 800-290-0065

If it says 223 on it then only shoot 223 thru it. The chamber pressure from a 5.56 is actually greater then that of the 223. Using 556 in a 223 barrel could resort in serious harm to urself or the gun. If the gun was bought from a gun shop an ordered by u thru them and u wanted a 5.56 then it is up to the gun store that u bought it from to correct the issue. Otherwise if u ordered the upper direct from ATI, and thats what they sent u my suggestion would be consult with their customer service line and see what options u have available. Do not shoot 5.56 from a 223 barrel but u can shoot 223 from a 5.56 barrel cuz like i said the chamber pressure from a 5.56 is greater then that of a 223 and the brass is also slightly thicker to a company the added chamber pressure from the casing