To create truly bullet-proof glass, you need to use two 1 inch thick brushes with glass texture on all sides. Each brush must be in its own entity (tested with func_wall and func_breakable) and no empty space can separate the brushes. That's it. So simple it's so easy to miss. You can use any combination of textures on any of the faces and not destroy the bullet-proof property of this configuration. Here is a sample map.