Should I get a Callahan or a Blitz Needle? Does the Callahan stop you from moving while firing? I just want a piercing handgun.
Callahan or Blitz?
New question to combat the fact that both stop you from moving: how long does a single Blitz burst go for? How fast is the firing speed of a Magnus?
You might want to check out the Lancer Knightz data on clips per minute and charges per minute. It will give you an idea of how fast these guns are, the damage per second that they can inflict (which isn't the whole story, but a good starting point), how much Callahan slows you down while charging, etc.
If you don't have ASI, Callahan and Blitz stop you for about an equal amount of time. Not that you ever really use the standard attack of Blitz; it's strictly a charge-only gun.
With a lot of ASI, you can get out of Callahan's animations faster than Blitz.
What are you trying to use a piercing handgun for?
Blitz has lots of dakka (I can't believe I just said that). If you want to kill a Trojan with a single charge or drown everything in bullets, get it. It has a high damage output if every bullet hits, but will take some careful training in order to use it without getting mauled. Devilites and Wolvers will be jumping around, although given how much of a clip this gun has, that may not matter.
Callahan has big bullets that knock down smaller foes and interrupts their attacks. However, its damage output is poorer, since it fires one bullet at a time and has a clip of 2 bullets. It's good for dealing with small groups of Gremlins/Wolvers/Devilites, but with larger numbers you're liable to get swarmed if you're not careful with it.
Should just get blitz needle. It has way higher potential than Callahan and can handle any non-piercing resistance mobs after you have mastered it, though the starting learning curve will be harder.
Here is a Blitz guide by Poopsie
http://wiki.spiralknights.com/Blitz_Needle_at_Clockworks
Bopp and Zeddy answered your question but it seems Callahan fires its clips faster than Blitz which means its firing speed is better and it doesn't interrupt your moving as much as Blitz. And I have something to add. Imagine a Callahan bullet hitting a T3 wolver. That bullet knocks down the wolver which means you can fire the second shot, even though your gun is slow and the wolver is fast. If you try this with a regular Blitz attack, the wolver will dodge the needles and spawn behind you, ready to attack. Callahan's stun will help you too, against those fast, dodging Wolvers and Devilites.
Wolvers and Devilites come in swarms, they usually don't come alone.
Charge the Callahan at those packs and they will be stunned. Then charge again and then again. You can stunlock them and charge 'till they die.
Well, one Blitz charge is enough to kill the whole swarm as those fast enemies hardly dodge charge attacks. But it's boring.
As a gunslinger I use both of these in the same loadout (though I use plague needle over blitz for personal preference) because their uses are situational.
If you're only going to carry one though you need to decide whether you want one that's effective at handling large swarms slowly but consistently or one that takes out individuals quickly and decisively.
It's also of note that the plague/blitz needle style guns are a bit harder to control because of the nature of the bullet spread and the stopping.
I would never ever use the two in the same loadout. Could be that I only use three weapon slots, but I really don't see the point of using both.
Blitz is simply superior to the callahan in both aoe damage and single target. As in, the blitz will devisively take out individual targets faster than the callahan.
The callahan is only safer when you really need that hit and run tactic going.
I find that, as I run the Arcade more, I've come to love my Callahan much more than my Blitz Needle as a general-purpose piercing gun if I don't find I need the Blitz Needle for something (and before you ask, they're both at lvl 10). These days I just pull out the Blitz Needle for FSC or OCH for the raw point-blank power against stationary or near-stationary targets and would never pull it out for any other reason.
Solo-wise, a Callahan can and will stagger-lock (at least when fully heated and properly geared) a regular wolver in place without giving it a chance to even teleport away. In larger parties it can seriously hamper their movement (but the bullet isn't fast enough to really sneak it in when a swordie is knocking a target around). I find that, against packs, the Callahan gives me the advantage of mobility (I'm an ASI-fiend) to give you those few steps to the side here and there to avoid blows as they teleport around you.
The only enemies I could REALLY recommend a blitz needle against on a regular basis are dust bunnies since they're the only real mob that properly clumps together consistently to really spray them down with the charge attack. Really, you find those in compounds and I find that I like to bring the callahan for the other non-dust bunnies hanging around there.
TL;DR: Callahan is a good, all-rounder piercing gun. The Blitz Needle is a specialized tool that's mainly valued for point-blanking things.
I want a piercing gun for beasts that allows me to move about quickly and allows you to at least not be helpless to attacks from behind while firing.
In that case, you may find the Callahan to be a better option. Wolvers in t3 have the unfortunate habit of burrowing all the time, so having a spray of bullets isn't quite such a great idea. (It's possible to use the Needles for this purpose by timing it to fire just before they attack, though. Just a bit riskier.)
Both stop you from moving. Callahan inflicts Stun and interrups enemies' attacks, while Blitz totally outdamages it. Take Blitz if you want super-OP-instakilling-overused weapon. Take Callahan if you want fun-to-use "hipster" weapon. If you're a PvP player: Blitz is OP in PvE but it totally sucks in PvP while Callahan is decent in PvE AND it can be DEADLY in PvP. Callahan is the Blitz needle of PvP.
EDIT:::Check Chawkthree's videos in which he demonstrates weapons -> http://www.youtube.com/watch?v=3FuCyhFvcUw (Blitz Needle) and http://www.youtube.com/watch?v=SS8ulw38Cgc (Callahan).