Veo here. It depends if you want to be "fair" or "effective". For elemental you'd want an AP or a Polaris (Or both, if you can). The Trollaris is kinda hands down the best elemental gun, to the point of being almost broken in some cases. But AP is more versatile, though it takes a bit more skill.

Shadow: Sentenza. Kinda hands down. You have two choices, that or The Umbra Driver. Go Sent. Wrecks gremlins. Fast like AP. No questions.

Piercing: Callahan or Needles, only options. Callahan is a hand cannon that leaves you so effing vulnerable it's not funny. Go needle. I use a Plague Needle myself, but that's a Krog Coin thing. Blitz isn't bad. But it has the same issue. Piercing will ALWAYS be your weak point, realize this. Devilites, as a pure gunner, are your bane.

Side note: If you want a good all-around normal damage gun, the Supernova Pulsar isn't bad. The catalyzers are gimmicky guns, only good if you have more than one in a group. The Valiance is always a good standby, and easily accessible, too.

It's got its uses. I just went through LoA and Polaris kept all those Almirian Knights at bay. AP couldn't do that. Same with Bombies. It's not the end all be all gun, but it does have some power to it.

Polaris has terrible DPS. It only knocks things back if you're far away enough, in which case you don't even need to knock them back in the first place (ironic?). Drivers are best for DPS if you figure out how to switch-shoot, Antigua line are good utility weapons and very versatile. Valiance is also good.

Can't really say what's the "best" gear, both ASI and damager boost are useful, it just depends on your personal preferance.

Polaris or Supernova are ultimate handguns of choice if you're pure gunslinger trying to get those pesky Devilites. Forget about autoguns and Callahan. Sure Devilites are vulnerable to piercing but it's not always about vulnerability. Plus those are great anti-turret weapons.

Umbra Driver is ultimate handgun against slimes and gremlins. The only place Sentenza is worth carring is Operation Crimson Hammer, since it is quite efficient versus gremlin mortafires. Charged Umbra Driver isn't bad against constructs or beasts either, but it's a close range handgun - it's best to think about any alchemer as a shotgun.

Elemental Alchemers deal great damage, but are a bit awkward in Candlestick Keep, where you need maximum mobility and range. That's why Argent Peacemaker is my weapon of choice versus undead. I usually supplement it with Voltaic Tempest or Ash of Agni, especially when in Graveyards or when Kats are involved.

Valiance is often under-appreciated, but I can't think of better gun. Probably it isn't best choice if you do lots of PUGs, but most weapons with knock-back aren't.

Blitz Needle is obviously ultimate Trojans and Lord V. bane, but since everyone and his grandma got one I wouldn't bother crafting it in first place.

Callahan used to have some uses in those time, when beasts were at least somewhat dangerous. Now you can kill beasts easily with almost everything so I wouldn't bother with Callahan. Mine is gathering dust and honestly I don't remember when I have been wielding it last time.

Long story short, I would recommend (in no particular order):

1. Umbra Driver - slimes, gremlins, beasts, constructs
2. Supernova (or Polaris) - devilites, turrets of all families
3. Argent Peacemaker - undead, silkwigs, constructs
4. Valiance - greavers, keeping enemies at distance when under pressure
then
5. Nova Driver - zombies, constructs
6. Blitz Needle - trojans