Blitz > Callahan > Biohazard > Polaris
To me most decent loadout for pretty much anywhere (most of the time i use this loadout just some time i switch polaris to magma driver outside fsc due knockback)

My very first long-term goal in this game was to get a gun of every fire type. I've completed that goal, and it's been completed for a few months now, which gives me a lot of personal experience with the ins and outs of each weapon type. Guns have a lot of variety and really shine in specific situations, so I'd never recommend not getting another gun, of any type (Iron Slug excluded :/ ) since that new one will help you in situations where the others won't.

The weapons that you mention are all good, with the exception of the pulsar. As I'm sure you've noticed, if you're a primary gunner, you want all your shots to connect to keep your damage up, and pulsars do just the opposite with their huge knockback. I'd recommend a Valiance instead: reasonably high damage, fast 3-shot clip, and just enough knockback to keep advancing enemies (like FSC zombies) at a respectable distance. A lot of players recommend pulsars because a lot of players are swordsmen, and need the heavy knockback to manage crowds or kill turrets; as a gunner, though, it's actually not the best choice.

I definitely recommend the Callahan, as well. There aren't many piercing weapons in the game, but the ones that do exist are CRAZY against weak enemies. You can bring pretty much anything down to a wolver den these days, but devilites and greavers are still as powerful as ever, and the Callahan rips them up while allowing you to stay mobile. I'm always the last man alive whenever I run fiend strata, because I can shoot, dodge, shoot while weaving around all the chairs and bats. As always, it takes a little longer to go through a level this way than with the equivalent sword (which I've also tried), but I find it much, much safer.

if you are having trouble with ur blitz then by all means get a callahan. but i strongly recommend practicing with your blitz more. once u know learn how to use it without getting hit, it`ll be your default piercing gun.

a nova driver is great, be sure to get one if you are going to keep on being a pure gunslinger. also practice with it so you can consistently spawn the ricochet inside your target (the nova can very easily out-dps a polaris). while we are speaking about the elemental alchemers, maybe you should also craft the status inducing ones. they are fun to use and they are definitely great for lockdown because of their high status inflicting chance.

if you have spare energy or is angry at your biohazard`s bullet speed, u can give the umbra driver a try.

don`t bother with normal guns if you are going to be a pure gunslinger; u need all the specialized damage u can get.

i got a collection of 5* guns (I don`t have the antigua lines, hail driver and all the normal damage guns).
my weapon loadout that i use most now-a-days:
nova, umbra, blitz

My usual loadout:

[ Slot available for anything ]
Nova Driver
Blitz Needle
Umbra Driver

I'm a charge gunner. Basically the idea is that with manual aim, just aim a little off to the side of your target and the drivers will ricochet into the target itself, dealing somewhere along the lines of 700+ (i forget) damage to a single target.

You can basically two-shot turrets in 4-player FSC, and in the case of that one rocket room, kill it and have time to block the rocket.

The Blitz is there because it's a gun that works very well when charged.

Even normal damage guns have their place, just not usually as DPS. Valiance in particular is the only gun in the game with its kind of knockback, and Neutralizer is a fantastic gun for soloing graveyards and FSC. Supernova has a very, very tight niche (where you need a pulsar, but not elemental or shock... some parts of IMF, maybe?), but is generally outclassed by Polaris, if you have to use a pulsar. Iron Slug is just bad, its place is in the garbage. :D

I personally use a Nova Driver, since it deals more raw damage. It's enough to reliably kill enemies one or two shots sooner, which is a big deal when you're doing ricochet tricks. Storm Driver is probably the next most useful, in my opinion, because you can rattle off another shot or two in the time it takes your enemy to recover from the initial shock, in both Lockdown and in dungeons. Umbra Driver's better in Lockdown due to the armor metagame, though.

I wouldn't bother with the Plague Needle. Autoguns are useful as boss killers or as horde sprayers, and either way, you're better off just doing more damage than trying to inflict a status, especially one as weak as poison. I recall reading elsewhere in the forum that you'd need two other blitzers to compensate for the damage you lose with the Plague Needle on Vana, assuming the poison is always on.

polaris, umbra driver, blitz, callahan. and nameless set

Yeah, l read the same thing about the Plaque Needle.

Also, agree with anti-Hail Driver comments. I have one, but the status, well.....It does no dmg unless the enemy thaws out, which is not gonna happen, as you're gonna hit it first.

Additionally, knockback is bad for dps.

Don't go Hail Driver; get Storm or Nova. Both of those can be used in FSC to optimum effectiveness, whereas with a Hail Driver you'll have to shoot slower. Shivermist and Glacius are better freeze weapons.

I'm not sure whether to recommend the Argent Peacemaker. It's a great gun in FSC (killing fiends and undead) but it's just average about everywhere else.