For those of you that haven't played the map, it has been made so that people can climb up on the Facility roof. There are also weapon spawns up there including slot 7, possibly giving the player a powerful weapon and continuous ammo.
In the classic version of the map, you couldn't even get up there. In a multiplayer scenario, people have been camping up there and picking people off on the runway.
My suggestion for fair gameplay would be to not allow people to climb up on the Facility. If you still want people to get up there, then to remove the weapon slots on the roof, allowing the player to stay there until their ammo runs out.