You know, I didn't think of this before but you might want to try a .38/.357 "Snake Load" ie shot cartridge. It would have basically no chance of actually injuring a bear (save maybe a lucky strike to the eye) but I would think they're probably more water tight than a blank round.

Glad to see you picked up your brass! grin
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman