Your pendant seems to be an inexact imitation of the logo on a US Army Artillery collar-insignia disc from 1910 to 1926, which had crossed cannons with the regiment-number and company-letter (but no "US" on it). Being inexact, your pendant would not be a Military-issued insignia or medal. I think Igarod's guess is on the right track... a Reunion badge, or perhaps what is known as a "sweetheart" pendant (manufactured for sale to soldiers' wives, daughters, or girlfriends, to show their connection and support for the soldier).
The photo below shows a 1910-to-1926 type of US Army Artillery collar-insignia disc, worn by soldiers in Company E of the 325th Artillery Regiment (or Battalion). You can see the similarity to what is shown on your pendant.