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8.8 cm Flak 18/36/37/41 - Wikipedia
What happened to all the flak fired at aircraft during World War II
Flak Definition & Meaning - Merriam-Webster
Anti-aircraft warfare - Wikipedia
Flak division - Wikipedia
Flak 36 88mm Multipurpose Gun - National Museum of the USAF
How Nazi Germany Introduced the FLAK Into the Military Lexicon
“Flak Was Our Worst Enemy”: Wilbur Bowers' Air War Over Europe
‘The flak can’t always miss. Somebody’s gotta’ die’
For Allied Planes and Tanks, Germany's 88mm Flak Gun