I think that duel damage is a good asset for all these things to have so please don't change them.
I also think that a pure piercing Antigua line is open for discussion as well.
But apart from that there is nothing else I have to add to what has been said already..
The issue with that is i am fairly certain they don't have weaknesses / resistances based on the specific weapon, just based on the monster, i.e every weapon gets a bonus from undead medium on undeads while Vana isn't classified as undead, therefore every weapon will not have that undead medium damage bonus against him. That is one example of a monsters weaknesses / resistances being based the monster itself and not the weapon. Second, I think what you are aiming at can simply be the exact same thing should it just be made pure elemental or shadow... unless what you are saying is you want it to keep the usefulness on gremlins and beasts with sentenza for example instead of the normal shadow's slime and gremlin? The reason you wouldn't want that is that you would end up having a gun useful against gremlin and beast, then say a callahan useful against fiend and beast, you've just over-written a family. And for what you said about people getting the guns for their intended reasons they are now... trust me, anyone who got an antigua line was expecting a lot, lot more than what they got -.- I remember when it was very hard to do JK in groups since it was toward the beginning of SK, I stayed up one night excited because I got a good group at last to farm out an antigua, just to get a gun that was incomparible to others.
Changing of the charge attack it should just be so you aren't locked, can aim different directions.