Bears are notorious for leaving faint bloodtrails. There fur is like a sponge and their fat plugs the hole many times.
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+2
+3 Especially Fall bears with lots of fat.
Also, IME many black bears drop at the shot -- no matter how trivial the bullet damage -- and then get up. One that my friend shot with a .270 had a small graze along his foot. The bear dropped, then jumped up and ran off, but succumbed to a second shot through the lungs.
