User blog comment:AugFC/An in-depth look at AW's weapon design/@comment-2163805-20141226213118/@comment-3967954-20141226220935

Yeah, the AMR9 does have a 5.56-sized mag, but I'd assume, given that it's categorized as an SMG and ejects SMG-sized cartridges (I think) that the front half of the magazine blocked off or something. I don't know; Sledgehammer's design team are bloody mental.