Hi,
It is a U.S. Model 1836 military pistol made by Asa Waters of Millbury, Massachusetts under contract with the US government. Waters operated a private armory that made a number of different US military firearms. Your pistol was originally flint and was the last flintlock pistol adopted by the US Army. In 1839, they sold for $9.00, I don't know what it is worth now.
dave