Literally anything hits the tail rotor and it's a spinning brick. .308, a drone, steel cable, Kevlar nets, whatever. Those things are hovering death traps

Nah, just about any rifle round can. It's a matter of hitting it somewhere important though. Most people can't hit anything at any meaningful distance. So now you're going to shoot into the air, miss the helicopter and take out someone a couple miles away as collateral.

A few cheap laser pointers from Walmart or the discount store and that thing will either be landing immediately, leaving the AO, or crashing violently.

But, you know, you should never do this. Or whatever I'm supposed to say here to prevent the secret police from kicking my door in.

The main rotor blades can take a surprising amount of abuse.
I found it even more surprising how much stuff can actually pass between the blades.
https://youtu.be/CiDqNRdxTO0

Not necessarily. If they start taking any small arms fire I expect they'll leave. 5.56 could crack the windscreen or injure the occupants. A bigger round would be more effective if you actually want to hit the engine or transmission and actually take it down, yes. You could probably do it with anything .300 or larger (7.62 NATO, etc.) if you hit it just right or multiple times. Or yeah go for something big like .50 BMG and hit it once somewhere in the meaty bits. But rifles in .50 are often bigger and more awkward to move around.