I would assume most aircraft/helicopters aren't normally shot down by a few rifle rounds. Aircraft/helicopters are too easily damaged and destroyed.

Also, when the helicopter is shot down, the propeller blades should not stop spinning so quickly. Helicopters use a technique called "Auto-Rotation" to land, where the main rotor blades disengage from the engine. This allows the upflow of air to spin the rotors, and when attempting to land, the pilot can pull on the collective and pitch the blades to make a softer landing using what lift the blades have left.