Probably the Remington Model 700's in a 308. I saw some photos of them in a post last year. Really good out to 600+ yards round drops about 8 ft. which is good enough in a urban environment. If they have rises from which they can shoot(mountain or high hill overlooking city), maybe the 50, but this sounds like in town.
.308? Maybe, but from what I have seen, the snipers prefer the .300 Winmag round. They had a documentary on the History channel about snipers and showed the Marine Corps snipers shooting 300 Winmags. There is a lot more casing volume to push a bigger bullet to the +3,200 fps range.