Even with 2 weapon slots you can always go for two different damage types; there'll just be two monster types that both guns will deal neutral damage to.

A gunslinger needs to have weapons to cover each damage type; pure gunslingers rarely use normal damage guns. The suggestions for each damage type are:

Elemental - Polaris, Storm/Hail Driver
Piercing - Magnus, Blitz/Plague Needle
Shadow - Biohazard, Shadowtech Alchemer

I put them in my personal order of preference. Pick one from each category and you're set, though Polaris needs a little work if you don't want to annoy your team (perfectly fine for solo play), Magnus and the Needles all need you to learn how to use them safely because they stop you moving.

As long as you don't carry all your weapons in one damage type, you'll be fine for any monster type. Magnus is preferable as your piercing gun mainly because dodging monsters won't try to avoid it - or won't succeed, at any rate. In fact, if you only have two slots and you want to be a pure gunner, you really want Magnus at all times once you hit tier 3 - less important before then.

I just crafted a Master blaster and got it to lvl 5. And so far i havent seen any change compared to the Super blaster i had. THe stats say its supposed to do more damage but i havent noticed any difference. Is that just me or has anyone else noticed that too??

For a long time I've been playing with two slots only and below combo worked great:

#1: Voltech Alchemer (upgraded to Storm Driver)
#2: Magnus (upgraded to Callahan)

Now, whenever I've decided not to buy 3rd and 4th slot, I'm playing with:

#1: Valiance
#2: Callahan

The above combo is the best solution for two slot gunslinger in my opinion. Just don't try it unless you have Valiance - it won't work with Master Blaster.

As for the Nameless/Justifier set - the difference between don't having ASI and having ASI is small one - about the same as having been bited by T3 wolver during reloading and knocking it down before it bites you...

Actually, the advice on this thread has been pretty damn terrible so far.

Just because you are using two slots does not mean you should only be going for two guns.

You should be going for three at least, one of each damage type, and then equip the two most suited for the stratum you are diving into.

For Piercing Damage:
The Callahan works magnificently against Devilites, Greavers, and Beasts
The Blitz Needle is superior to the Callahan only in Firestorm Citadel vs Trojans and Vanaduke himself (jlwfnord, I'd like to see you fight a mob of 10+ greavers with the blitz needle in T3).

For Shadow Damage:
You have basically two choices here, the Umbra Driver or the Biohazard. I tend to favor the Biohazard because of its 3-shot clip and the ability to make gremlins dodge continuously just by firing a single slower bullet. The charge's AOE also wrecks Jellies. Also, the Biohazard can cause poison.
On the other hand, the Umbra Driver does not do more damage per single target shot than the Biohazard and only really has an advantage in faster bullet speed.
If you have the energy/funds for it, get a Biohazard

For Elemental Damage:
Again, you have a choice between any of the Elemental Drivers or the Polaris. I favor the Polaris, because the knockback is extremely useful, and it is ridiculously awesome in Firestorm citadel. However, if you'd rather inflict Freeze or Fire, maybe the Hail and Magma drivers are more for you. But in a choice between Storm Driver and Polaris? I choose Polaris.

First - you might add me ingame for asking everything or even party if u want :) Abathur

If you got 2 Slots,
you should take the Magnus and Pulsar for Piercing and Elementar Damage. This is very important, because the Magnus is also a perfect weapon for Gremlins, because you can stunlock Menders (permanently let them fall back and dont let them escape or interact). For Slime you can use Pulsar. Also the Wolvers and Devilites won't avoid your Magnus shots, and even if they do, they get the damage. The Pulsar Series' expanded bullets can prevent Gun Puppies from attack. If you hit them while charging, they stop their attack. This is a very important point of the weapon and also something i have to tell you later at the armor thing.

If you got 3 Slots,
just get a Toxic Catalyzer. Gremlins will avoid the slow bullets well, so you anyway use the Magnus, but en Mass you can pin with the charge attack bullets to them, dealing massive AoE damage. Also this weapon can inflict poison wich makes it much better than the alchemer, while dealing same damage with lower bullet speed but +1 Shot per Clip. Poison is great against Mendors, Silkwings etc. Prevent Healing, decrease Atk and Defense by 50% and 10%. Best slime destuctor ever.

If you got 4 Slots,
add Blitz Needle. You can (with Charge) oneshot any Mendor, devour also his protection bubble, you can oneshot Trojans or even Mass of Devilites. It's a risky weapon, but like all other not alchemer weapons, you need to learn to use them.

As Armor, you should take the Nameless or the other Armor that name i forgot but also does increase the attack speed. It's pretty important to have a good attack speed. It does make pretty much sense if you know how to shot. It's anyway awful to get 2x low damage with ***** ...
How ever, just do shot with Pulsar and Toxic Catalyzer 2 times instead of 3. So you can increase you DpS greatly. You won't reload, you shoot faster than other gunslinger - way fast. Attack speed doesnt only mean the speed between shots, it also means the speed how fast you grap your weapon and bring it back in normal position. However it does not increase reload speed.
With the nameless Speed Bonus you can stunlock T3.2 Gremlins with Magnus, and you will be able to prevent 2 Towers from attack with Pulsar. No way, ASI is useless on guns. They who say that don't know how to use guns or are using just alchemers ...

mfg Abathur - feel free to ask

@Xyonon

I disagree with your assessment

My own IGN is Jamini. Same as my forum name.

If you have 2 Slots
You should take a Blitz Needle for Piercing and overall damage, and an Alchemer (Hail Driver or Nova Driver depending on your play style) or Polaris for elemental damage. Covering two damage types is vital, and the Blitz needle has overall the best DPS of any gun. Outside of slimes and constructs (which both die very readily to Elemental damage) it can chew through pretty much anything quickly. Grivers and Devilites both go down extremely quickly (the former do not dodge, the latter will not dodge the whole attack and WILL get inturrupted by any damage). Wolvers and Gremlins can be a problem, but all guns suffer against wolvers. A Callahan will not stun a wolver/gremlin 100% of the time, even with a Max damage boost, and both the Magnus line of guns and the needle line require you to stand still, making the Blitz a better investment overall.

3 Slots
Get a Toxic Catalyzer. Biohazards rip through slimes like nobodies business, and do reasonably against gremlin-only groups (just drive then against a wall.) The poison is useful, and it does have more damage per clip than an Alchemer (though you should be using them on monsters they are weak too in the first place.)

4 Slots
Get a Bomb or Sword. Sometimes it's inadvisable, impractical, or just plain stupid to use a gun. (See, T3 gremlin danger rooms) for those times, you will want a backup weapon. Personally, my fourth slots regularly swaps between my Shivermist Buster (Excellent for soloing Vanaduke), A Dark Briar Bomb (It's like a Callahan... except with a massive AOE and chain-able. I highly recommend it for wolver danger rooms, even without bomber armor), and a Leviathan Blade (No resists, excellent damage, nice safe charge. What's not to like?)

If you absolutely must run x4 guns, then get a Supernova or a Valiance.

For armor, I recommend Nameless or Deadshot for the resists. ASI is fairly useless for gunslingers (except for autoguns and antiguas) but the damage reduction from the associated sources is very noticeable. For trinkets, Trueshot is a godsend (CTR can be very useful for Alchemers and Autoguns if you prefer that).

It is impossible to stunlock T3 wolvers with nameless. Even with x2 Trueshot trinkets a Callahan will only stun wolvers roughly 66% of the time, and Gremlnis even less. Quite honestly, if you run into them your best course of action is a well-placed bomb or a few sword charges. Anyone who tells you otherwise probably doesn't solo danger rooms very often (if at all).

If you want more info -
IGN: Jamini.

@yjun73:
Yea Justifier Set is ok. It's nearly the same as the Nameless one. You also can take the Deadshot one for FSC, but overall ASI are the most useful gunslinger armors. DMG Bonus low/low is just laughtable, also you get weaker against curse (doesn't matter) and poison ...

@Jamini:
I know the Blitz Needle can handle wolver, devilites and even greaver. Single greaver are easier with the Callahan, but it doesn't really matter. But for devilites, you can kill a couble of them with the Blitz very fast, while you need some more time with the Callahan, but if there are many of them you are lost with Blitz. The same happens with wolver. You never can stay still for a shot, and if you can, it's so rarely that the DpS is lower than the Callahan's one. I personally ♥ both :D
ASI is important as you know: Sometimes the Callahan pulls somone back (Gremlin) so he can't attack, this happens not all the time, you're right, but the 2nd shot allways does that effect. Thats why ASI is important. Also you're faster able to move quicker again if you don't have to reload. You also deal much more damage and are more flexibel while using Polaris and Biohazard.
ASI doesn't affect Blitz Needle's shot. It counts as one shot.

I would like to add you ingame for some duo runs in T3. As long there aren't mass greaver, I'm happy :D (IGN Abathur)