I’m a pretty radical pro-2nd Amdnedment type. I think that private citizens should have access to everything a Light Infantryman would... at minimum!
Have serious doubts about anybody ever using a .50 cal rifle to knock down an airliner. Maybe you could disable one on the ground with a few well placed shots, but taking out a flying one would be a heck of an achievment in marksmanship.
I think a .50 caliber could set a 747 on fire while sitting on the runway with an incendiary round quite easily. You don’t have to hit it in the air when it is essentially a rolling fuel tank with people on board.
If one were to get in the right spot at a busy airport I would imagine you could do tremendous damage with a rifle and cartridge designed to destroy light skinned vehicles.
I don’t imagine we will see a publicized test any time soon but from my time with Uncle Sam and the shooting I have done with .50 caliber rifles as a civilian I would bet you could light a commercial aircraft on fire quite easily.