It takes roughly about 30 rounds to shoot down a helicopter with an assault rifle, regardless of the barrel length and whether the weapon is suppressed or not.
But with a short-barreled ACR, it takes 90 rounds. With a standard-barreled ACR though, it takes 20~30 rounds.
Strangely enough, this doesn't happen when Tier 1 mode is on. In Tier 1 mode, you can shoot down a helicopter with a short-barreled ACR firing 30 rounds or so, both with a suppressor and off.
FYI this bug only applies to damage to aircrafts.
Hope the devs fix this because the ACR is one of my go-to weapons.
* Check that. Damage to ground vehicles is off too.