Looking at WS damage only, those
AGI+20...
* ...add 20 * (2 + fTP) base damage (as in before magic damage modifiers) to Leaden Salute.
* ...add 20 * 7.5 = 150 base damage to Wildfire.
* ...add 0.85 * 20 = 17 base damage to Last Stand.
Totally ignoring those implied
RACC+15. Because.
To make math easier (it's past 0600...), I'll just assume MAB+100 in gear and using Leaden Salute at 1200 + 250 TP. Why 1200? I'm not logged on to check where Leaden Salute + pewpew would put me at, TP-wise, so I'm generous here.
Putting Armageddon in the gun slot against Vanir Gun (the most viable non-RME pewpewboom gun, I think) does this:
Armageddon: Base damage + 20 * (2 + 5.215) = Base damage + 20 * 7.215 = Base damage + 144.3
Vanir Gun: Base damage * 1.05 = Base damage + 0.05 * Base damage
2887 base damage is required to make Vanir Gun better for Leaden Salute.
2887 = 2 * dAGI + 5.215 * (2.45 * (152 + 119 - 99) + AGI)
2887 = 2 * dAGI + 5.215 * AGI + 2197.601
689.399 = 2 * dAGI + 5.215 * AGI
You literally have to have really low AGI (for today's standards) to have Armageddon beat Vanir Gun on Leaden Salute.
Shortened math for Wildfire:
3000 = 2 * dAGI + 3.3 * AGI + 2317.7
682.3 = 2 * dAGI + 3.3 * AGI
It's a lot closer for Wildfire, but more often than not, Vanir Gun beats Armageddon here, too.
I'm too dozy for Last Stand now, but making up for the remaining 7 base damage that Vanir Gun is behind seems to be more likely than not, if we pewpew for TP.
Does AM change anything? Most likely not, because getting AM means you "wasted" damage on Wildfire instead of Leaden Salute or Last Stand already. ODD would have to actually catch up that damage first.
Long story short: I wouldn't make an Armageddon i119 as of now. Especially if you want your gun to be among the top two weapons for pewpew.
[rant]
Not that it affected Wildfire (or Leaden Salute) damage here, but SE overdid it a bit with keeping i119 Emps weak, just because of ODD. If they changed Empyrean AM3 duration to something long, maybe things might be better, but right now, I cannot think of one single i119 Emp that's significantly better than other much more easily obtained options.
Oh, well. At least my mannequin has something to play with while I'm gone...
[/rant]