It will be good either way since 150 is a lot of Extra ATK or Mastery ATK. It will boost Slug Shot at the very bare minium of 18,000~36,000 depending on the size of the target and that isn't even including ranged modifiers.
And as you say it provides a Kyrie Effect which is neat :U.
Gatling Fever definitely gives 120 Mastery ATK so I will assume it most likely is also Mastery ATK.
I think it is eATK.
Using a Crimson Bolt and Purifying Bullets I do 7.2k with fire dance.
Using a water converter puts me at 8k with fire dance.
Then I remove the endow and do Crimson Bolt with Purifying Bullets with 10-coin Platinum Altar -- 9.5k with fire dance.
Using a converter on that then puts me at 11.5k with fire dance.
I was using no other gear at all so the only 'hard' atk I got was from the gun, the bullets, and the platinum altar buff.
(The damage values are an average of 3 fire dance casts.)
Edited by mildcontempt, 06 December 2017 - 04:39 PM.