The effective anti-personnel range of any rifle is the lesser of a) the range at which the bullet drops from supersonic to subsonic, or b) the range the shooter can hit an 8" circle consistantly.
Regarding a), this transition causes a shock wave to knock the bullet around randomly, destroying what had been until then a smooth ballistic trajectory. A .308 or .30-06 for example generally reaches that point at about 800 or 900 yards, depending on the specific bullet.
Regarding b), the limiting factor is either the rifle/ammunition combination or the shooter. The rifle is only as accurate at the person shooting it in most cases. A good .50 BMG is as accurate as a .308 or .223, so the limiting factor is how good the shooter is. If a person can shoot 2" groups at 100 yards with an accurate rifle, then that's the limit, irrespective of the bullet size or the muzzle energy.
The vitals of the human body are generally considered to be 8 inches across. That means that the shooter's maximun effective range under ideal conditions as a sniper is 400 years
regardless of rifle type.
In order to be able to take advantage of the extremely long range potential of a .50-caliber round, a person needs an extremely accurate rifle and the ability to use said rifle to shoot tiny little .25" groups at 100 yards, which translates into an effective range of some 1,600 yards, about a mile.
This is an incredible distance, more so because environmental and gravity factors become hugely important the further down-range the target is. At those distances, knowing the exact range to the target is vital because gravity is starting to really take it's toll on the bullet's path. Being wrong by 50 yards either way can result in a miss, as the bullet either flies over the target's head or hits the ground at it's feet.
Not only that, but you have the wind to contend with as well, not only where you are, but every where along the bullet's path. Remember, it only has to blow the bullet to the left or right by five or six inches to make you miss. How much training do you think you'd need to be able to do that?
And is your target moving? That compounds your problem even more because flight time can be well over a second at extreme ranges.
.50 BMG rifles are very expensive, thousands of dollars for even a single-shot one. The kind of scope that has enough clarity and power and durability for a .50 BMG also costs over a thousand dollars. Practice ammo costs $3 a round, and the good match stuff even more.
And what is the potential terrorist getting for their money? A half-inch hole put in something. Shooting a gasoline or propane tank makes it leak, that's all. Like leaving a valve cracked open a little. Shooting down an airliner? Please. Maybe with a .50-caliber machine gun. Next to the runway. Assassination? Except for the President or Vice-President, there is no person in the country that you cannot get close enough to to use an ordinary rifle with a much higher chance of success than a .50 at a mile.
And, finally, after California banned the .50 BMG in 2004 new alternatives emerged. One is the .510 DTC, which fires the same bullet at the same velocity using a slightly different cartridge, and the other is the .416 Barrett, a .50 BMG brass case necked down to a slightly smaller bullet.
It's such a fringe issue it's not even worth pondering.