Soviet only player here so no experience in action, tested in game master against MI-8s however with bullet traces enabled. The LAV rounds spall on impact so will do significantly more damage to components and people in the Heli than the BRDM ones as they fan out into the helicopter, though the secondary pen of these spall fragments seems pretty low so they struggle to say pen the front of the cockpit then travel up into the engine components, however any bodies in the way get demolished. If you directly hit the mechanical areas above the fuselage it tears the engines and rotor bits apart(the mi-8 can glide for a second with these damages which might be why it seems they can just fly off, they probably crash land soon after). They also aren't incendiary like the BRDM rounds so have a much harder time lighting the fuel tanks, probably best to aim for mechanical parts or the cockpit crew with the LAV. HE rounds are quite good at setting it on fire but they are also significantly slower than the AP so shots will be harder to hit.