As always, read the label before buying. Some of them are biodegradable. They melt when hit with water. That might prove adverse to your types of deployments.
No one yells at me to go shooting in rainstorms these days, but your mileage may vary.
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Good question. I have bought my bags, and prefer the Triad Red Tac with poly beads for rear bags. But the best fill I've found comes inside the Wiebad Pump Pillow. I've love to know what's in that bag.
http://cdn7.bigcommerce.com/s-xh035r...537084.jpg?c=2
According to a 6.5guys article it's Micro Bead. AKA bean bag chair fill.
If you are not carrying them very far, truck to bench, I'll second or third the Sinclair heavy bag sand.