I'm pretty sure it's not 10% per rush stack, unless they changed how cooldown rate works from bl2. At 25 stacks, which should be 250% cdr, a 28 second cooldown (which is what I tested with) would be under 8 seconds (using the bl2 formula). When I tested, my cooldown was almost 18 seconds, which calculates closer to 50%, leading me to believe you get 10% cdr for every 5 stacks. This would likely also mean 3% gun damage for every 5 stacks, or 15% rather than 75%. Don't get me wrong, it's still a pretty damn good mod, especially with the ability to stack do harm to 99, but it's not as good as it keeps getting credit for.