I would contact STI to be sure. My 5.56 Oly Arms would eat both (.223 and 5.56) but I was warned that a .223 match chamber may cause issues with 5.56. rounds. The 5.56 chamber throat being ever so slightly different to a .223 chamber throat. But STI will know for sure what their chamber is.
The guys at sportshooter.co.nz should know-they live ARs.
Cheers.
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