When the barret was first introduced to service, it was against Geneva to use it against people. Its primary use is anti-vehicle (or EOD), just like the KSVK and Boys AT (just to name a few) rifles that preceded it. It is useed to shoot the tracks on a vehicle, a tank or BTR, and if the round hits the tracks, the tank/apc is disabled, and it cant move anywhere. since JO has no vehicle hitboxes, I just made the barret destroy the vehicles.
http://en.wikipedia.org/wiki/Barret_M82
As for the light vehicles, JO doesent support having a round do differnt damages to diffrent items. (will do the same damage to Hummer or Tank)