I guess we all have different perspectives. To me, the OP statement ". . . for use when bear spray doesn't work." means when it is very windy. (Though I'm not sure why he'd be fly fishing then.)

Clearly it would be difficult to change defense plans in the midst of a bear attack. Things happen fast. A friend of mine surprised a sow griz with cubs and before he could react she had his leg in her mouth. Bear spray in the eyes made her let go and run off with the cubs. Having a gun-shot and dying bear clamped on his leg would likely have resulted in much worse injury for him. He was alone and hiked out several miles after stopping the bleeding - no broken bones. Obviously, some luck was involved, too!