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Rickie, the barrel is stamped 5.56NATO and will fire both 5.56 and the lower pressure .223. If the barrel is stamped .223 then the higher pressure 5.56 rounds should not be fired.

Put it this way; if you get a firearm chambered in 5.56 you can shoot .223 Remington out of it. It is NOT recommended to shoot 5.56 out of a firearm chambered in .223 Remington. The different pressures and the scientific equations of the cartridge and rifle will likely cause problems to the firearm and possibly the shooter and people around the shooter. In a way, it is kind of like shooting +P or +P+ ammo out of a firearm that is not rated for any type of +P ammo. It was my pleasure to assist you.

Rickie, Not with this firearm- S&W M&P 15 Sport ll. It will handle 5.56 and .223 rounds. Some firearms that are chambered for only the .223 round can not safely handle the increased pressure of the 5.56 round.

I have this particular firearm. It will shoot both rounds. The caliper size is stamped on the barrel and says 5.56 Nato. If you get the opposite gun with .223 stamped, it is advised not to fire the 5.56 rounds through it.