Hello, I am a pure gunslinger that occasionally uses bombs so my recommendations might not meet your needs perfectly.
Elemental guns: There are three types of elemental guns, however I believe swords can do much more damage (and DA and brandish series are more powerful because of its reach on charge attacks.)
Polaris: I would not recommend this, especially with the new update, causing more popularity of arcade and exploring the clockworks. This weapon is not team friendly, and the knockback and bright flashing is very very disruptive to your team mates to the point that many Polaris users are kicked as soon as they join. It is a viable weapon, but I highly suggest you don't spam it everywhere and think about others when using it.
Argent Peacemaker: This is a perfectly good sidearm (meaning support not high DPS). This allows you to shoot while retreating and kite enemies before you finish them of with your main damaging weapon (in which case yours will be a sword). Also the charge attack is useless and gets you killed.
Alchemers: As a gunner, I believe that this is a great DPS weapon, because of its ricochets and charge attack. However, as a sword wielder you might find it lacking with only two bullets. I would recommend this to deal status damage and charge attack for a non-gunner. This is not so friendly in switch heavy levels and bomb block heavy levels because of its ricochets that can easily get you and your teammates killed. Also this weapon becomes much more stronger every time you upgrade it for its number of ricochets increase.

Piercing guns: There are only two piercing guns that are both extremely situational and root you only firing. My advice would be to get a piercing sword for fiends and wolvers in general.
Magnus: Don't get this if you aren't a pure gunner. Unless you are able to switch-shoot this, it will just get you killed.
Blitz needle: I suggest you get this for the King of Ashes mission, and Trojans, for this is a heavy DPS weapon. It can drain Lord Vanaduke's health and kill Trojans in one hit.

Shadow guns: There are three shadow weapons, but like elemental guns, they can be easily out damaged by swords.
Catalyzer: It is a unique and fun gun. It is like a sticky bomb launcher. The charge attack shoots an bullet that orbits an enemy that can be detonated with a normal shot. The bullets are slow like a pulsar's and are not so useful against gremlins. However, it is very useful against slow, non-dodging and clustering enemies (compound thingys). Inflicts poison
Sentenza: Same as Argent Peacemaker
Alchemer: Same as its elemental versions, does not deal status.

Normal guns: Normal damaging guns are very useful and handy.
Supernova: Same as Polaris
Valiance: Very handy that deals small knockback on all of its fast bullets. It is very handy while retreating, and is a great sidearm.
Neutralizer: Same as the shadow catalyzer. Unlike the shadow versions, it can be used against zombies, and are quitte effective against them.

Also there is only two Antigua lines: Elemental and Shadow, there are no piercing lines.

Honestly I would reccommend AP or Polaris. Alchemers I wouldn't recommend because its better as a primary weapon than as a sidearm. Gunners can acheive a very high DPS with alchemers because of the charge and bouncing bullets. As a swordsman, I think you need a sidearm for when you need it or to kite monsters. You are not going to think about which side of the monstor you are going to shoot and you will he mainly using the gun's basic shots. Alchemers have great charges. But you will rather use the swords charge because you are a swordsman. Hope this helped.

Alchemers are great guns, but Polaris and AP/Sentenza are better sidearms than the alchemer. I don't really suggest you get a blitz until you get your first 5* set and can choose whatever to craft.