I believe its an Artillery collar disc with the fastener missing. Scroll down to Type 5 (V) here: Dating Metallic Insignia: Collar Disks
I'm uncertain about the dimple above the crossed cannons... Don't know if a particular state or company letter(s) might have been attached there?
U.S. Army Ordnance Corps buttons? Yours seems to have been a two piece. Based on a French design.
U.S. had a lined field that is not seen in your pic..