Re: LE .50BMG questions.....
The purpose of a .50 in law enforcement is the same as in the military: To engage targets at ranges beyond the effective range of smaller rifles and/or targets that smaller rifles or pistols would be ineffective on. If your at an airport you could easily have an engagement range of 2000 yards in some circumstances(lack of cover, not enough time to get closer, multiple points for a suspect to enter or leave). And any jurisdiction is going to have vehicles or objects that pistols or .223/.308 caliber rifles would be ineffective on.
Having a .50 is no different than having a radio, handcuffs, baton, Taser, and a pistol on my belt. It gives me options to deal with a situation in an effective manor. If someone steals a vehicle(regular or a heavy vehicle) and won't stop, without a .50 the only means I have to deal with it is using a spike strip or shooting out the tires with either my pistol, rifle, or shotgun and hoping the driver decides to stop or does something or something happens that causes the vehicle to loose mobility without further endangering people, or try to shoot the driver and hope the vehicle stops in a controlled manor. My department had a guy steal a 18-wheeler when I was in high school and the only reason it was stopped was after the officer's shot some of the tires out, the guy lost control and ran off the road into a field and got stuck. If I have a .50 and the circumstances allow it, I can back off and put a .50 round through the engine or the differentials. Take either of those two out and vehicle stops and most likely no one gets killed or injured.
Say you have an active shooter holed up in a building or behind a wall or another object a .223/.308 rifle can't penetrate effectively and there's no cover near enough for you to get a shot on the suspect with a .223 or .308. This youtube link gives you a good idea of the relative penetration capability of the rounds against normal types of building construction:
http://www.youtube.com/watch?v=lprGoEpDX...PL&index=23 . Purely from a liability and cost stand point, a single shot from a .50 rifle is a much better choice then the full auto fire from .223 or .308 machinegun that would have to be used to replace it.