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I have often wondered how much those Ur sacks are useful against a bear.  A friend had one on a mountaneering trip, and his crampons "got out" and destroyed the Ur Sack... Bears teeth have to be worse than crampons... right?

Another thought I can't get out of my head is the food will be completely crushed and not very edible once a bear is done with a sac.... 

Am I wrong?  Missing something?  Any real world testimonials out there?