I would think that one of the last things even a professional sniper would use is a .50cal. Sure it's powerful and has long range but...
1. How often are you going to need a 1000 yard rifle in a city?
2. Are you trying to shoot through armor? A .308 AP will go through body armor or armored galss.
3. These things are huge and conspicous (sp)
4. There aren't a lot of these things sold. It wouldn't be too hard to track down one sold legally. And if it's sold illegally....then the law wouldn't help would it?
Even for a trained killer, it makes much more sense to use a standard caliber rifle to kill with. This isn't a battlefield with targets of oportunity. The hit would be so planned out that the target would be much closer. I just see it as completely impractical.