It is indeed a US Army collar-disc insignia, with crossed rifles designating the Infantry branch of service. Because the emblem on yours is manufactured separately from the brass disc and then "applied" onto the disc, your collar-disc dates from 1937 to the present. However, that date-range can be narrowed a lot by examining the "form of attachment" on the disc's back (how it was attached to the uniform). For info about the various forms of attachment, go here: Dating Metallic Insignia: Collar Disks
Please post at least two photos showing your collar-disc insignia's back, shot from different angles.