The M53 AP which get's fired from the M61 on the M163 Anti-Air variant of the M113 need's the be buffed.
When trying to shoot down multiple Mi-17's you start to notice that not only it takes to many rounds until the helicopter goes down but that the helicopter goes down because you shoot out the Pilots and not the helicopters systems like the engine or rotor. I'm pretty sure that 20x102mm API with only a few good hitting salvos should easily take down any helicopter available in the CDLC. Meaning the engine block or the rotors should be destroyed much easier. Whether this "issue" (IMO) is because of the damage values from the ammo itself or because of the damage model from the Mi-17, this needs to be tweaked accordingly.

Also the ammo should be called M53 API instead of M53 AP because if it actually is the M53 it's Armor Piercing Incendiary and not just Armor Piercing.