A 50 bmg could be a very good terrorist weapon... the only prob is it would be very hard to use it as one.
The shooting planes out of the air, is a joke, you might be able to hit one on take off or landing, but thats not going to bring the plane down unless you can take out more then one engine( not going to happen). The other way it could be used as a weapon of terror is to attack either a chemical plant, refinery or storage tanks... once again it would be extremely hard to do that. In all of those structures the only really weak parts are the pipes and valves. its very hard to hit a 6" pipe at 1000 yds, let alone hit it at the right angle. the valves are only a little bigger. All of this hinges on know which ones to shoot... There are alot easier ways to do the same damnage if not more. In both cases you could do alot more damage with a light aircraft then you could with a rifle.
Every other target that a terrorist could shoot with a rifle, could just as easily be done with something much smaller, a 308 would be more then enough. Hell, most of the targets a 223 would be enough.