I'm pretty sure that the sentenza path has increased dmg to slimes, reduced to undead.
Argent Peacemaker has increased dmg to undead, reduced dmg to slimes.
The antigua's dmg has no reduced or increased dmg, so it should do about the same dmg to every monster, which makes it stronger against slimes against silversix, and better dmg to undead to the blackhawk. The silversix does better against undead and blackhawk does better dmg to slimes
Note* this is my opinion
Jellies are weak to Elemental, so they should take more damage from the Silversix than the Blackhawk.
But since I don't own a Silversix, I can't say.
But once again, dual damage weapons aren't additive, their damage actually gets nerfed separately, so instead of adding those two bars together and going, whoa I should be dealing twice as much damage, you're really getting about the same damage split into two types that have two separate strengths and weaknesses.
That's where the whole damage thing comes in, your shots added together would be better, but they're not.
So when it rolls against higher armour, you deal less damage than your previous weapon because the damage is split into two lesser forms when it hits.
I think that's what the problem is.
I know one of the devs said there was an issue with dual damage weapons before.