The gun that is used the least in every loadout is the Antigua lines. Why? Because all other guns deal much more damage than the Antigua counterpart, from 3* to 5*. While I can understand Antigua is supposed to be a "Mobile" gun, just about every other gun is a "Mobile" Gun. Autoguns are supposed to be committal of course, but they are still the best gun to take in nearly every instance.
The largest problem that Antiguas face is not only on their damage, but also the fact that their charge attack is the absolute longest in the game. The bird at the end is exemplary, however, as the Bird can pierce through groups of enemies before exploding. In comparison to other lines and how quick their charge shots are, it begins to put out the question: What do Antiguas need Buffed?
1) Damage. The Damage per shot is relatively low compared to all other guns. Given that the understanding is that six shots means you get to shoot more, this damage gets equalized in comparison to things such as the Blaster lines even when you're shield cancelling to reload. This especially goes for the most recent weapon, the Mandolin, which already sports rather high damage. A single shot from an Antigua is equivalent to a single Autogun shot, and this should not be the case.
2) Charge Shot. The Charge Shot on Antiguas is the worst in the game because it roots you in place and causes you to fire 5 shots quickly, but then a bird that pierces through enemies. This makes it difficult for it to compete with other guns' charge shots, which makes some sense on the Antigua being a "Mobile" gun - but it also runs antithetical to its identity. A big change for Antiguas that could help keep it useful is to only have the Charge Shot be the bird at the end, removing the previous shots. This gives it a similarity to the Magnus Lines, but it allows it to be more versatile.
3) Damage Up Vs. Gremlin/Undead/Fiend. Damage against Monster Families is relatively undertuned for these weapons that are supposed to fill a specific niche. High may help, but this doesn't help much given "1) Damage" explained above. In most situations, this is a negligible increase. In order for Family Damage to matter, it needs a significant increase either to the Passive or Base Damage.
4) ASI and Antiguas. ASI does not have any use for Antigua given the speed of shots as is. At most, the Antigua line does not benefit from CTR either due to how lackluster its Charge Shot is. For the most part, Antiguas only have a reason to rely on utilizing UVs for Monster Families that they either are already effective against or are neutral to. (i.e. Argent Peacemaker is effective against Undead(High). Adding Construct(High), Fiend(High), and Slime(High) are the only options an Argent Peacemaker can take since CTR and ASI do not contribute to its damage)
In Summary, the Antigua Line is one of the Weakest Guns in the game since it does not fill a single niche, not even status wise(where Catalyzers are concerned) exempting the Obsidian Carbine. If there's any way that this gun can be reworked or changed to make it a viable gun compared to the likes of Autoguns, Alchemers, Magnus, and the Mandolin which are used heavily now, it'd help diversify Knight builds instead of always pushing only the Autogun line to the forefront because of its high damage output.
I'd prefer changes that give the Antiguas a situational advantage instead of just boosting damage.
Pallaskat suggested somewhere that antigua shots could pierce through multiple enemies, which is clever. Maybe the shots could deal more damage with each subsequent enemy that it travels through.
Can get rid of the useless damage bonus, everyone has bk anyways.
ASI does benefit a small degree, faster shield cancelling.
Agree with charge shot, I remember suggesting the same thing some time ago just being the bird.
Similar to the above, maybe the charge shot could deal increased damage with every subsequent enemy traveled through, including aoe splash damage.
Or maybe each charge bullet could be a smaller crowd piercing bird instead of a basic bullet, and ends with the big bird. Still locks you in place as a tradeoff.