ok, this is a bit old, and it might have been answered on another post, i dunno. But the best way to make bullet proof anything in the source engine, is the texture named "block bullets" put that on a brush, and bam, no bullets go through. For glass, make two brushes with the glass texture on them, and then simply texture the inside of the glass textures with "block bullets" and bullets will not go throguh, same for walls.
Note, i thought I was under the source forum... lol... dont know if hl1 had block bullet.... lol, my bad, my bad....