It is definitely the silver-plated front of a 2-piece US Army Infantry button, from the 1830s/1840s. Your button-front cannot be dated more closely than that, because its back is missing. To view its specific type, see buttons GI-82 and GI-83 in the button-book by Alpheaus H. Albert, "Record of American Uniform And Historical Buttons - Bicentennial Edition."