Everything Ravenna said.
The sim treats everything as a machine gun bullet, & only tracks it for 2 seconds. So "real world" muzzle velocities won't give Heavy AA the reach it needs, but speeding up the bullet makes them super-accurate.
The only other thing I would add to Ravenna's comments would be to tweak the angle-off:
[guns.0]
gun.0=1800,0,2.4036,1.1808,0,0,1
change to:
[guns.0]
gun.0=1800,0,2.4036,1.1808,+0.5,-0.5,1 or similar
This means the initial rounds will be high(+) or low(-), or starboard (+) or port (-) of the target's real position. The AI will try to compensate, & as the range decreases & the angle off error distance reduces, eventually it will get onto target, but this buys you engagement time.
I use this quite often for bomber guns, my rule of thumb is adjust both for hand-held weapons, & one or the other for turretted weapons.
Also I would checks the time between rounds.
[GUNSTATIONS]
gunstation.0=4,-1,1,10.00,1127.76,60,0.08,4000,0,100,1d1*50,0,0,0, 0,0,0,-180,180,80,-10,8
as too often these are theoretical rates of fire, & flak guns in particular would not operate at these rates all the time (they wait for the first rounds to burst & correct the barrage).
Also don't forget many light weapons are clip fed, so sustained rates of fire are far short of the values often quoted. ie increased time between rounds. eg JAP 25mm auto actual engagement RoF was 225 - 250 rpm, far short of the guns capability, mainly because of the clip feed issue.