WASHINGTON — Sen. Jim Inhofe squared off with Defense Secretary Robert Gates on Thursday over the decision to cancel a U.S. Army cannon that was to be partially assembled in Elgin.Inhofe, R-Tulsa, said he disagreed with Gates’ decision to cancel the cannon and other proposed manned ground vehicles that are part of the Army’s Future Combat Systems.At a hearing of the Senate Armed Services Committee, Inhofe told Gates Congress had mandated a cannon production schedule.