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The .223 is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO military cartridge. ... Due to its lower pressure, you can safely fire .223 ammunition in a 5.56mm chambered gun; however, the same cannot be said in reverse. The 5.56x45mm ammo creates a higher pressure.

Thomas, it can fir either without issue. Most 223 is loaded to lower pressure than 5.56. The exterior (physical) dimensions of .223 and 5.56 ammunition are effectively identical. 5.56 ammunition may be loaded to higher pressures than .223 ammunition. 5.56mm chambers are dimensionally larger in certain critical areas than .223 chambers. Given the same ammunition, 5.56 chambers will have lower pressures than .223 chambers. ---- https://www.luckygunner.com/labs/5-56-vs-223/

you can fire either or even mix them in the same magazine you can fire both in guns marked 223/5.56 or just 5.56 but do not use 5.56 in a gun marked .223

Thomas - Mix 'em, Match 'em ... alternate them ... whatever you want, because you can safely fire either or both. Buy whichever costs you less for paper punching. The 5.56 NATO round is what he military uses - it has just a bit more power, but they are essentially very close in size and power. Having a rifle chambered for 5.56 you can fire both. were your rifle chambered in .223, you could not shot 5.56 in the rifle.

Thomas, yes Sir. Any firearm stamped with 5.56 Nato alone or with 223/5.56 Nato will fire both by default. If only stamped 223 then it is recommended firing 223 only Sir. This is due to the shorter 223 chamber that could cause over pressure with the 5.56 round.