NOTES
-There are no barrels in Levels 1 & 6.
-Barrels can be knocked around by Special Infected.
-When a player destroys a barrel, a text message appears in the chat. But if something or someone else destroys a barrel, there will be no message. However, it will still count.

MAP 2
1: On the right, right outside the saferoom.
2: On the highway.
3: When you finish crossing the bridge leading away from the crane, look up and to the right. Hard to see; use the flashlight.
4: By the stairs in a room in the sewer area, across the little bridge.
5: On the right, right after passing through the flood gate.
6: On the left, a good bit away after passing through the flood gate. Can't miss it.
7: In the warehouse leading to the safehouse, on the lower floor.
8: Same area, but on the lower floor.
9: In lower floor inside safehouse, DON'T CLOSE THE DOOR!
10: Same area as 9.

MAP 3
1: Right outside the saferoom behind a couple of barrels.
2: Behind the gate across the wooden bridge.
3: Right behind 2.
4: As you walk up the wooded path past the river, look for a small cave that the barrel is tucked away in, just before reaching the road.
5: When you reach the fork that leads to a right & left path, check the right side of the fallen tunnel.
6: Near the tower on the right path.
7: At the truck garage behind the fence. It's next to a car with alarm, so shoot it at your own risk.
8: At the last area before the safe room where you need to break the barricade.
9: On a tower in the same area, look up.
10: On the roof of the overly tall safehouse, look up.

MAP 4
1: Near the generator by the examination room, under some stairs.
2: In one of the warehouses when you get outside where all the trucks are.
3: By a unopenable door in the same area, one without its handle.
4: Same area, on a platform. Look up.
5: Hidden behind some crates above some wooden stairs that break apart, straight ahead before the path on the right leading towards the lift where rocks fall and break the metal bridge, right after turning the valve to stop zombies from pouring out.
6-9: At this point you'll come across two paths. However, it looks like one only path can be chosen per game. Luckily, barrels won't spawn on the path that isn't open. Worst part is though, is that these four barrels are in random locations...
10: On a platform near the safehouse. Can't miss it if you've taken the left path.

MAP 5
1: On the left right when you leave the safehouse.
2: Right by the lift, can't miss it.
3: Behind some crates on the path forward, after the lift reaches its stop.
4: In the locker room inside the building.
5: Look for one of the blue metal porches and step onto the one where you can see the other part of the building, with all the glass. On the roof is the barrel.
6: On the roof by the tent where the two Witches are.
7: Under some stairs on one side of the room where you have to start the pump.
8: Same as 7, but on the other side of the room.
9: Behind the big rig after opening the giant door near the lift, near the safehouse.
10: ???

If you've found a barrel or if I made a mistake, let me know.