Hey guys,
(from 'The Bombing War: Europe 1939-1945' by Richard Overy)
During the Battle of Britain and The Blitz, over the night campaign from July 1940-June 1941, the UK AA guns were considered to have shot down 170 aircraft with another 118 damaged.
In September 1940 alone over 260,000 AA rounds were fired. The Air Ministry estimated that the number of AA shells per shootdown began at about 6,000 per aircraft in the autumn of 1940, reducing gradually to 3,195 to April 1941. Improved radar, improved/increased searchlight units, and improved organization and methodology further reduced the number to 1,830 per shootdown in the spring of 1942 (the best for the UK prior to the introduction of the proximity fuse).
(from 'Archie, Flak, AAA, and SAM' by Kenneth P. Werrell which can be found on the dtic website)
(
https://apps.dtic.mil/dtic/tr/fulltext/u2/a421867.pdf)
(re UK AA effectiveness)
Starting in October 1940 the UK began deploying radar for aid in shooting at night. The rounds per shootdown went from 30,000 in September 1940 (when German night bombing began), to 11,000 in October 1940, to 4,199 in January 1941.
Against the V-1, the proximity fuse was considered 5x more effective than timed or contact fuse.
(re German AA effectiveness)
In the ETO the USAAF credited the German AA with shooting down 7,821 out of 18,418 aircraft lost on combat sorties, with 6,800 lost to enemy fighters, and 2,179 lost to operational or unknown causes.
After the war the US did a study that found if the Germans had used proximity fuses, the heavy FlaK would have been 3.4x as effective. This would have made operations by the B-24* "impractical" and operations by the B-17 "very hazardous".
*My note. The 88mm FlaK 18/36/37 made up ~60% of the German heavy AA during WWII. The effective ceiling of this gun was considered ~26,000 ft, about equal to the the practical operating altitude of the B-24.
(from 'German Ground-Based Air Defenses, 1914-1945' by Edward Westermann.)
The Germans estimated ~3,000 rounds per shootdown in 1943.