[Brophmeister]

the effective celing of the 88 mm(flak 18) was about 7,900 m. check wikipedia
Comparatively, the 88 was not as powerful as its Allied counterparts. In the anti-aircraft role it fired a 9.2 kg (20.3-pound) shell at a muzzle velocity of 790 m/s (2,600 ft/s) to an effective ceiling of 7,900 m (26,000 feet) (at maximum 10,600 m, 32,000 ft).early models.

88mm flak 41
Rheinmetall responded with a new 88 mm L/71 design with a longer cartridge. It fired a 20.7-pound shell at a muzzle velocity of 3,280 ft/s (1000 m/s), giving it an effective ceiling of 11,300 m (37,000 feet) (maximum was 15,000 m, 48,000 ft). It also featured a lower silhouette on its turntable mounting

Wikipedia is dubious, at best, but even so this is still higher than the apparent maximum effective altitude of AAA pieces in game.