Author's Note: This is likely to be a bit more contentious, I'd wager, because everyone seems to have a different vision of what a true secondary is. Bear with me, and make sure to comment!So you've picked out a primary blaster and are ready to bust some foam up in a G's grill. Wow, you've suddenly become gangsta. Maybe you should hold the blaster the right way?

In all seriousness, the reason for secondaries is simple: primaries fail. That lovingly-modified blaster you spent an inordinate amount of time working on will, eventually, chew up a dart, run out of battery juice, or just plain run out of ammo. When that happens, you have two options.
A: Go feed the zombies.
B: Pull out a secondary blaster and resume filling the air with foam.
I am assuming, gentle reader, that you would rather maintain your higher-brain activity and as such would choose Door Number B. If you're choosing A, why are you reading this, anyway? Go read something by Chev.

When choosing a secondary, mind you, things get more complicated. It is best to have a secondary that compliments your primary, but deciding on
how to complicate things is the tricky bit.
Using a "Primary" as a SecondaryOne of the first things that pops into a lot of people's minds when thinking about a secondary blaster is "hey, I can just grab my Alpha Trooper and use it if my Rayven craps out on me." Personally, I would recommend you skip that line of thought altogether. It's entirely possible to do, but the blasters that people tend to use as primaries are larger and bulkier than I would prefer to try to quick-draw and fire.
There are two exceptions to this rule, in my eyes. Due to its small size (as a pistol), relative power, and ammo capacity, a Recon makes a wonderful secondary, and likewise a Raven can be used as a functional one as well. That said, if you're using a Rayven as a secondary, I'm a bit curious as to what your choice is for primary.
Wacky SecondariesSome people, perhaps calling themselves Grenadiers or Mad Scientists or whatnot, like using something silly as a secondary. I'm talking about the Scatter Blasts, Spider-Man Wrist Blasters (or wrist-mounted Jolts), a Titan with a 19-round absolver attachment, and so on. These are all lots of fun, but I'd recommend carrying them as a weapon of last lulz. Take a serious secondary in addition to the "fun" blasters and chances are you'll have more of an opportunity to use the goofy ones.
IntegrationsOh, how I love these. At the start, it's one easy way to use a primary as a secondary. Just attach the two together, somehow! Integration gives you a secondary (or tertiary, or quaternary) weapon ready-at-hand whenever it is needed, no slings required. So long as your primary has a suitable shell, the sky is the limit. However, there are certain blasters that make for easier integrations, so let's go over them at this time.
Night Finders, Jolts, and SSPBs make for the easiest integrations due to their small size and simple mechanisms. However, you'll notice that they are all plunger-fire and muzzle-loading, meaning they are pretty much SHTF blasters. For those of you who don't get the reference, I'm sorry.
The undisputed king of integrations, however, is the Swarmfire, particularly when combined with the Stampede to make what is perhaps unimaginatively called the Swarmpede. Even on a non-electric blaster, having a Swarmfire integration can be a wonderful idea for when you really need a FA spray to get things done.
Blowguns also make very handy integrations (and can go into just about any primary). It will give you a long-ranged potshot weapon that's handy for breaking up charges before they start. If you want to be efficient, it's entirely possible to mount a blowgun to a tactical rail, allowing you to move it from blaster to blaster.
The SwarmpistolA Swarmpistol is, effectively, the guts of a Swarmfire mounted on a pistol handle. They are typically powered by a battery pack placed in a pocket and connected to the pistol through long wires. If you have one of these or know someone who can make one for you, don't bother reading any further--you have your secondary.
SnapfireThe Snapfire is as of this writing unreleased, although SGNerf has obtained one and written up a review. If the review holds true, this is the go-to secondary for most new players, straight out of the box (although I'd at least remove the ARs). A true semi-auto blaster with 8 shots is just about perfect, in my eyes.
Okay, that was a bit of a mouthful. Let's move on to the criteria I will be judging potential secondaries by.
Size/Ease of Draw: Carrying around two bulky blasters isn't much fun. It is important for your secondary tool to be small, lightweight, and easily-accessible.
Rate of Fire: The higher the better. If you're drawing your secondary, things have already gone south.
Ammunition Capacity: Again, the higher the better.
Range: A very minor consideration here, since in my view, your primary should be used for longer-ranged shooting and your secondary as a bit of a panic button. Still, better to be able to tag earlier.
X Factor: Anything special brought to the table by the blaster.
Note that, by this time, I am going to make a few assumptions:
1: You have read through the Primary Guide and chosen one that has good range and RoF. Something like an Alpha Trooper or Rayven, likely.
2: You know how to perform at least simple modifications on your blaster. I'm not going to discuss purely stock secondaries. My reason for this is simple: if you cannot perform those mods, chances are you're not going to have a situation where you can get a secondary out before you're nommed.
Best Tier secondaries aren't going to be particularly surprising, I don't think. They consist of the following two:
Socks
Swarmpistol
Integrations
Melee
Hey, wait, socks were listed as a primary, too! Frankly, yes. Socks trump blasters everywhere but fun. If you aren't carrying a few dozen in a pocket or two, you're doing it wrong (note: I often do it wrong!). The Swarmpistol and integrations have been discussed earlier.
Melee weapons are amazingly useful if they are allowed at your game. If so, get one and learn how to use it properly so you aren't breaking Z noses.
Not going to suggest mods for these, since the Swarmpistol is completely modded and integrations are a whole area of modifications. You can tie-dye your socks if you want. The Zeds might appreciate it.
Highly Recommended secondaries provide a lot of versatility and can both get you out of trouble and help cover the weak points of a primary. They are these guys:
Recon (Pistol Configuration)
Barricade
Snapfire
PotentiallyBoth of these (not counting the Snapfire) can shoot relatively quickly and hold at least 10 darts, both of which are major advantages. The Barricade is noisy and has a short wind-up before firing, and the Recon requires two hands to fire, so they are not without their own faults.
Suggested mods:
Recon: OMW kit/AR removal and spring replacement. The OMW kit really makes the Recon into a scary secondary.
Barricade: Li-Po batteries, taped flywheel, dart holder (so the ammo doesn't fall out when the blaster is pointed down).
Snapfire: Remove the ARs.
Recommended secondaries are still quite good but need a bit more work or have a few more faults. Here they are:
Rayven
Magstrike
Firefly
Spectre
Absolved pump guns
The Rayven's compact shell really helps it as a secondary (but it makes such a great primary, as mentioned before). With the Magstrike and Firefly, they're capable of rapid-fire but are just a little too big for my taste. The Spectre gets good range and is a great size, but its low ammo count hurts, as does the required two-handed operation.
Absolvers get a special mention here. You can carry them pumped and ready to go and then spray out multiple darts when needed.
Suggest mods:
Rayven: See the Primary discussion
Magstrike: AR removal for a start. Extra mags would be a good idea.
Firefly: I genuinely have never messed with one, so I'm not certain.
Spectre: Remove the ARs and replace the spring.
Not Recommended blasters just don't quite have what it takes to get things done. Note: they can be used as hold-outs or last resort weapons, but I wouldn't recommend them as a secondary.
Maverick
Nite Finder
Jolt
Scout
Reflex
With the exception of the Maverick, all of the blasters are single-shot muzzle-loaders. That's just a recipe for disaster as a secondary. A Nite Finder can actually be used pretty well as a potshot weapon (and is great for integrations on funky blasters like a BBB), but I just don't see it as something I'd want to have if I were in the thick of it.
So, that's my thoughts on secondaries now. Anything I missed?
