Hi Lowell, taken from the CZ-USA web site FAQ page -
Q: “Can CZ rifles chambered in .223 fire 5.56 ammunition safely?”
A: FOR US BASED CUSTOMERS- All of our .223s will happily eat 5.56. Since our factory is in Europe, we build everything to CIP spec, which doesn’t differentiate between the two cartridges and just has the higher pressure as its standard. So the CZ .223s will shoot everything from the cheapest Russian steel to match .223 brass ammo.
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Read it yourself here (2nd question) >>> https://cz-usa.com/support/faq/
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Stay safe & have a great day!

Even though they state .223 they produce the barrels to withstand the higher pressure of the 5.56 rounds as well.

Because it is.

Hi Lowell and welcome. Most .223 firearm metallurgy today can withstand 5.56 pressure. There is a disconnect that boils down to the way .223 and 5.56 pressures were measured when in actuality, they are closer in psi when the same measurement is used for both. Cheers, stay safe and enjoy!

Because they also say on their website: "Q: Can CZ rifles chambered in .223 fire 5.56 ammunition safely? A: For US based customers - All of our .223s will happily eat 5.56. Since our factory is in Europe, we build everything to CIP spec, which doesn’t differentiate between the two cartridges and just has the higher pressure as its standard. So the CZ .223s will shoot everything from the cheapest Russian steel to match .223 brass ammo."