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• n  artillery designed to shoot upward at airplanes

• The Italians on the shore had depressed their ack-ack guns.
• The Indian air force had knocked out the Pakistanis' runways and, out side of the limited range of ack-ack guns, Indian planes could fly as freely as if they were at an air show.
• Here's heavy ack-ack now loud firing and muffled shouts of crew'.

• antiaircraft
• antiaircraft gun
• flak
• flack
• pom-pom
• ack-ack gun