[Imp]

Just guessing but the reason it is the only helo that behaves that way is because the penetration value for the HE is so high.
HE does not lose penetration over distance
This means the HE ammo is better at killing hard targets than the dedicated AP ammo
Try changing the HE ammo Pen to 2 & I bet it uses the AP ammo, thats still high most A/C have a HE kill of 1 I think.
As I said numbers may have got a bit muddled as what I think from a quick look represents the plane mounted version has a Pen of 6.
As planes use HE Pen this may have been carried over to the helo weapon by mistake, there were a lot of recent changes.
Pure guess here but I would say HE Pen of 1 possibly 2 & AP of 8 for the helo version.
This is based on the fact the helo version has a much longer barrel.
On this point think the Accuracy ratings might need looking at unless planes recieve a modifier. Currently 22 plane 20 helo but helo weapons barrel is 1/3rd longer. Muzzle velocitys 880 vs 960 but plane has a higher rate of fire.
All just a quick look so could be talking out of my xxxx
Just loaded MBT quicly later model russian helos use a slightly longer weapon with a lower rate of fire. This is represented by kill being smaller but Pens are HE1 SABOT 8.
Accuracy was 23 I think, sorry no weapon ID edit time running out