Arsenal Credit Union (ACU) is a state-chartered credit union headquartered in Arnold, Missouri, and serving the St. Louis metropolitan area, chartered and regulated under the authority of the Missouri Division of Credit Unions. Member deposits are insured by the National Credit Union Administration (NCUA), a U.S. government agency. As of April 2014, ACU has approximately $200 million in assets, more than 25,000 members and six branches. It is the 10th-largest credit union in the St.