I finally got around to testing damage reduction today, and got some very strange results.
Using Deadeye bots in solo advanced pve (consistent damage with low rate of fire) I got these results:
288 -base damage against Montana
202 -with only basic hailstorm
272 -Blissbeast skull plate (5.6%)
134 -Blissbeast and hailstorm
68 -BB and mansformation
72 -hailstorm with icy resolve
36 -hailstorm and mansformation
18 -hailstorm with icy resolve and mansformation
17 -hailstorm + icy resolve + mansformation + Blissbeast
173 -base damage against Toby
121 -with panic mode
165 -epic damage reduction gear (5.05%)
115 -panic mode with gear
note: Panic mode is only active while the orange indicator is on the screen.
It appears that stacking damage reduction on Montana halves incoming damage for each skill or helix. Activating damage reduction gear triggers this on basic hailstorm, greatly increasing its effect.
This does not work for Toby, where the DR bonuses are instead multiplied.
Is this halving effect specific to Montana? It appears unintentional, but only Gearbox knows for sure.