I think he's right, 4% or 6% would be favourable. Tightrope's testing seemed to show that the Quad was less reliable than the OKW Flak Half-Track or Centaur (Side Note: It also showed the M15 AAHT is completely broken and doesn't shoot half the time, this should be investigated.).
We can easily further finetune the chance percentage. Though I would like to see a bit more tests or do more myself if I can find the time. AA chance is very RNG, so only ~2 tests may not show the complete picture. At 20 ROF with 2% chance per bullet it should theoretically still have a pretty high chance
every second to kill a plane on average. We could try 3%.
I will ask to look into creating a new weapon for the M15's AA mode that can turn 360 degrees to see if that fixes the tracking issues, but I don't have modding experience so I don't know if that is possible.