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Hi William, This will Chamber both 5.56x45mm NATO and .223 Remington.

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William, the Sig M400 Barrel is "Stamped" 5.56 so will fire both 5.56 and 223 ammunition with out concern.

the Sig M400 Barrel is "Stamped" 5.56 so will fire both 5.56 and 223 ammunition with out concern.