I just find it unlikely that they are using anything odd like a PPC 22 or a 22 BR Remington. Although it wouldn't surprise me if they had a heavy barrel varmint type rifle from one of our leading manufactures.
My fear is that they could have a sniper fire support team with them for the one time they get found out. One sniper and a truck load of guys with AR's. They might have other vehicles with scouts using cell phones to keep track of the cops before they shoot. To catch these guys the cops are going to have to cordon off a hit immediately after it happens, and search every car and person. I think they better be ready for a big fire fight.