BudsGunShop.com - barrel has 223 not 556 stamped on it

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

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

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

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 : americantactical.us/contact.html

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