Bulletproof and hardness are different things. One exists the other doesn't.
Nothing is "Bulletproof". The proper term is bullet resistant, as in it can resist small arms fire. About the only thing you could even consider bullet proof is multiple feet of reinforced concrete buried underground. Remember, no matter how resistant it is, there is always a bigger bullet. Most of the bullet resistant armoring sold to heads of state or light military armoring can't even stop high power 50 caliber rounds. It is my understanding the presidential limo has armoring and windows that can stop a 50 caliber round, it also weigh like 12 tons. And just about nothing short of a tank can stop 2 inch or larger rounds.
Once you get up to the 6-12 inch rounds there isn't much of anything that can stop them but the solution I mentioned above. Once you are up to that size the energy that needs to be stopped is just beyond what most materials can withstand.
Armoring is constantly evolving to increase resistance but nothing is bulletproof. Not the vest the cop is wearing nor the cars sold to heads of state.