> Everyone seems to be an expert today.
Pot meet kettle?
Speaking as a former helicopter gunship weapons and tactics instructor (WTI), this is a VERY broad generalization. Sure, a gunship can shoot down a drone with 20mm or 30mm but you have to get pretty close. And first you have to find it.
Other factors 1. target altitude 2. air superiority (MANPADS is a real threat over land) 3. marksmanship
It might be more effective to throw AIM-9s on helos and target drones that way...but depending on the generation of missile those run ~$300k each and the inventory is limited.
I have heard the Ukrainians are using APKWS (https://en.wikipedia.org/wiki/Advanced_Precision_Kill_Weapon...) against drone to good effect from ground and air platforms.