How the armored doors are divided into classes
Armored door can have 13 safety classes. Instead of steel, special metal sheets are used, which are made of bulletproof alloy. Such metal must be checked to comply with GOST.
For the first and second security class, it is enough to use a sheet of armored alloy 2 mm thick. To increase the security class, thicker sheets and various special fillers are used. No bullet can break through such a door, explosions are not even afraid of it, unlike steel.