The shield being a "raised" one, with a wide border, and the eagle having more-horizontal wings than a civil war era eagle-button, it appears to be an 1874-to-1902 US army Enlisted-ranks (Private, Corporal, Sargeant) button.
Does it have a backmark? If so, please post a close-up photo of the backmark.
I see what your looking at Gunrunner. Some buttons have gold guilting still on them but this either like you say is paint or some form of plastic coating they put on them. Also a great reason hunters should never clean things in the field.they may just rub off some history
Great find. I still have never found any sort of button. Maybe some day. Great find!!!