Stopping power

Back when Woody Allen was starting out, on the stand-up circuit, one of his best-remembered jokes recounts how he was walking along a street, minding his own business.  It was one of his foibles that he carried a bullet in his breastpocket.  At that moment, from a window high above, a preacher threw a Bible at him, which bounced off the bullet in his breastpocket.  "And that Bible would have gone right through my heart if the bullet hadn't been there."  As a secular Jew in the counter-cultural 1960s, the punchline had added poignancy.
Of course, it was a riff on all those anecdotes about how Bibles in people's breastpockets had prevented stray or intentional bullets (in everyday American life) from killing the holder of the Holy Book, as if this was a divine nod from Providence.  Tonight, on Last Word, the obituary programme on Radio 4, Stephanie Kwolek was being remembered as the inventor of Kevlar, which not only stopped bullets in flak jackets but has been applied in a whole multitude of ways from aircraft landing wheels to the casing around fibre-optic cables.  Yet, naturally, the greatest fame was for all the lives saved of people who were wearing Kevlar in one situation or another.
I've thought for while though that if Bibles had such stopping power why not make a bullet-proof jacket out of them?  Sure, it may be a bit bulky but I'm sure if would cheaper than Kevlar.