I had noticed just earlier today how terrible my Grenade's damage was sometimes. Considering it wasn't consistently terrible (It could still reach the high numbers of my range), I am lead to believe that Grenade is unaffected by Gun Mastery, in that it does not cap it's minimum damage at a certain number. I had left Grenade at Level 12 (Where it does 100% my damage) to best make example of this. Here we have me using the skill on a snail, and landing significantly lower damage than the minimum of my range. This should not be the case when Grenade is meant to do 100% of my damage, and snails have very little defense.
Make that Blank Shot, too. It's been managing to do less damage than any single bullet of my Double Shot when it should be doing more than them.