The Bullet resisting glasses made by laminating alternating layers of glass and PVB interlayer to form a single multiple-ply glass laminate. This combination works because the glass layers smash and flatter the bullet, absorbing the shock of impact. The interlayer holds the glass together keeping the glazing unit integral to some extent absorbing the impact energy. The minimum thickness of Bullet resisting glass is 25mm in which you can have 12mm+PVB+12mm or 8mm+pvb+8mm+pvb+8mm shown in the diagram below. The more layers makes stronger and the maximum thickness you can have up to 65mm.