AR-15's shoot 223
Edit: I know they can shoot many chambers but I find 223's most pleasent.
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Their main chambering is actually 5.56 Nato, that chambering can also fire .223, it is however not smart to shoot a 5.56 from a rifle chambered in .223.
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An AR-15 can be chambered for .223 Remington, 5.56 NATO, 300 Blackout, 6.8×43mm SPC, 7.62×39mm and 5.45×39mm. Aftermarket parts are available for additional calibers, up to .50 Caliber rounds. - Der
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Decent AR's are 556 it is a hotter load, it is what the military uses. The AR must be chambered in .556. Never shoot a .556 thru a chambered .223.
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They actually shoot many different calibers.