Yeah. You are completly right. I've checked it in mod tools. Pretty much all planes have 1000 armor and all AA units have a modifier on deflection to either insta destoy or do nothing to plane. Oswind for instance has 24% chance to destoy it and Quad has 10%, Centaur 16%, M15 and OKW HT 17%.
All other AA guns in the game have projectiles even Centaur, while Quad dont. Maybe its connected to this or ROF or something else. But its just most likely just works uncorrectly.
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It probably simply has to do with the sheer volume of fire the quad delivers that 4 barrel of MGs firing.
The problem started when it was patches and its AA capability was increase by around x33 times, which is a crazy buff.
M17 Quad
We wanted to tone down the performance of the Quad to be in-line with Light Tanks and other suppression platforms. In order to make infantry counters to this unit viable, the M17 can no longer suppress enemy squads on the move.
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Increased the chance for the Quad to take down an airplane (from 30 in 10000 to 10 in 100)