[Travis McGee]

A jet engine at the starting end of the runway coming directly toward the shooter would have virtually no relative motion, it would just appear to grow in size.

Easy shot.

[TheRealLobo]

You said:"A jet engine at the starting end of the runway coming directly toward the shooter would have virtually no relative motion, it would just appear to grow in size.

Easy shot."

Maybe an easy shot. HOWEVER, your shot would have to be REALLY lucky to just damage the engine enough so that four minutes later, catastrophic engine failure occurs. If you shoot it while it's coming towards you on takeoff and catastrophic engine failure occurs while the plane is moving towards you, you are likely to find yourself in (part of) the debris field. Could someone be THAT lucky? Sure, I guess so, but I can't work through the amount of problems that this scenario would present.

There are far easier ways to bring down an aircraft than trying to do it with a .50 caliber rifle.