h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Caution: oxymoron in progress, yes this one is right up there with Army Intelligence folks and on the same shelf as bulletproof vest. 'Bulletproof' glass will only stop low velocity high calibre rounds or shrapnel from a blast. It will not stop rifle rounds or the humble rpg... How do I know this? I have seen it done, .357 magnum, .45 colt, 12 gauge shotgun, none of them went through, although they made mighty impressive damage... we had a .308 (7.62mm) to test with too, it went through like a hot knife through warm butter. We didn't have a .223 (5.56mm) rifle handy, but considering the muzzle velocity is almost three times greater than in the .308, I think it would be a safe bet that you wouldn't be safe on the other side.

Now police snipers may closer fire their shots, but it is not solely for that reason. Let's face it, we are dealing with a small object moving at high speed (and decelerating), you also have the glass to contend with. Two shots is security, its a lot like skipping rocks on water, except the rock will kill (maim) you if it hits you and two, you can't throw a rock at 1000+ feet per second (unless you are in the Chinese women's athletics team), you have to get the angles right to get the rock to bounce (or in this case not bounce). You can't always guarantee this, so what do you do? Give that man a cigar, you use two shooters (or yes, two members from the Chinese women's olympic athletic team).