Not sure if anyone's done this before. People probably have, but I'm just putting it out there again.
To make a three-shot combo with the Magnus, fire a shot and hold attack like you're charging an attack. A second later, release. You will fire another shot. Then click again. Another. No reloading except after the third. Nifty, eh?
Magnus Three-Shot Combo
Usually with the Callahan (or any Magnus line gun) it's best to shield cancel straight after each shot. Since Wolvers will teleport/dodge when you shoot, you'll shield bump them away from you. And since you stand still to shoot, you attract Devilite attacks, which you can then shield.
@Extribble
This allows you to attack faster, though. Reloading is the main weakness of a Gunslinger, but it's not that big. Extending your combo to delay reloading can be more effective than avoiding it altogether and therefore lowering damage output.
EDIT: I also know that tactic and have used it. However I find it extremely useless for two-shot weapons (and unfortunately I find myself using only two-shot guns for some reason. It seems all the decent three-shot or higher ones are boss token rewards or just too expensive for me. On a side note, I'm working on getting a Pulsar and/or an Antigua), as it halves the damage output. With this, you gain an extra half of your normal damage. Therefore, if you compare this extend-clip tactic to the avoid-reloading tactic, you gain 100% extra damage, technically, if you still count the normal clip as 100%.
As a heavy Callahan user here I have to agree with Alpha here, this is useful, I've been trying it myself and it does work well, I just have to remember to pause for that one moment after the first shot instead of just habitually rapidly clicking.
If you time your shots right, you never have to reload with any gun. You shoot the number of shots in your clip minus 1, pause until the animation just finishes, and shoot again :P It's been well known for quite a while without doing the whole partial charge-attack thing you mentioned.
So by this method, you can pretty much have an infinite combo with any gun :P