Not until ArmA 3 will we see .50 rifles disabling the engine blocks of cars. However, that round will go through certain light armored vehicles with ease, and kill any crew who are sitting in the way. This works best with ACE mod, of course.

So far as choppers go, you're best off hitting the pilot, as most components take multiple .50 rounds to disable. Tail rotor and main rotor engine block are valid targets for any 12.7mm weapon.

Edit: And if your Chernarus nights are pitch dark at midnight and early morning, be sure to run without beta patches. They're experimenting with the gamma.