[Imp]

Okay misunderstood thought you were talking about their weapons not their armour.
Yes there is a problem with armoured air you seem to need warhead size 6 minimum to cause damage.
Versus a heavy armoured helo (4) probably warhead 10 plus.
Armour 4 baring a lucky hit is immune to all MANPADs and virtually all None Area Sams like the Patriot
Warhead size seems more important than HE
15 HE 10W probably the minimum to cause a couple of points of damage won’t kill it.
70 HE 20W overkill normally 30-200+ damage from my observations.

Do a test turn experience off and set to 60-100 helo-Sam side. Edit EW to zero for helos to improve hit chances. Possibly increase hitting never tried that.

Regarding AA guns firing AP rounds at air targets have you never watched a war movie with Flak?
You fire HE at air and hope it gets hit by the blast because your chances of actually hitting the plane are remote.