For techies only. U.S. contract solicitations indicate the same basic method used in WW2, the “Upsetter method”, is employed. The Upsetter forge is a horizontal forging press.
A Signal Corps documentary explains the Upsetter method in great detail, minus the automation of the modern Scranton plant:
MANUFACTURING 3-INCH ANTI-AIRCRAFT SHELLS WORLD WAR II ORDNANCE FILM FRANKFORD ARSENAL 10414