772ed185_Grand_Theft_Wiki__the_GTA_wiki__Description
[Ability] Bullet-proof [Code] BP [Description] 
Cars being bulletproof typically means the car is immune to gunshots. To test if a car is bulletproof, the player can shoot it with a firearm many times and if there is no smoke coming from the car, then the car is bulletproof.
Special Vehicles - Grand Theft Wiki, the GTA wiki