You absolutely do not need .50 to penetrate an aircraft skin. A
regular 7.62 NATO, not even AP, should do nicely. And there is a whole
lot more MG's in that and similar Warpac calibers that are out there,
than 50's. And there are AA MG's too. That to me would pose a greater
risk of actually bringing a liner down. Though you have to be a lot
closer than a click. But in most cases you can be directly under the
flightpath of very low flying jets. Shooting a 767 with a .50 to get a
hit in the midsection somewhere, seems like a waste of time.