This is mostly for Stellar Set but also because I want at least one.
Here are your only choices:
Nova Driver
+Allegedly good damage, splits
-no status, I tend to use sword/bomb for elemental
Callahan
+ Knocks down wolvers and gremlins, fast, good vs vanaduke, may stun
- I already have Radiant Sun Shards, Dark Briar Barrage, and Dark Thorn Blade.
Biohazard
+ I hear it's pretty good. Plus it's unique
- other players may mess me up, and so far toxic catalyzer seems really weak and slow.
Polaris
+ Great all-around, very popular
- Doesn't seem that much better or different from Gigawatt Pulsar, given the cost...
Blackhawk
+ Unique, overlooked weapon, best gun against gremlins and wolvers, and useful in pvp
- Doesn't seem as useful elsewhere, especially when I already have a lot of piercing weapons.
Just a few things:
Callahan not so good vs vanaduke. You're thinking blitz needle
Biohazard has a learning curve. The slow bullets may miss a lot if you're only going to use it when you have low hp and you are backed far away from mobs.
Polaris has higher damage than Gigawatt pulsar, most weapons work like this...? I don't see a reason not to upgrade it if you like it.....
More importantly, what are you using this gun for? What else will you equip with it?