Okay... that didn't really go anywhere. Let me try this: What sorts of characters to do think of when you hear the word "gunslinger"? More specifically, who are some progressively more impressive gunslingers? As a matter of fact, I'll make it even easier: Who are some amazing marksmen? While I'm looking for guns more than anything, being a good marksman can transfer quite well. Here's what I have so far.

For Fundamental tactics, it's already fairly obvious; the best place to look is at exceptionally talented soldiers. It's easy to imagine a real person pulling off these kinds of feats. Heck, even low level bullet time is possible considering how people sometimes describe their responses to high stress situations. It also explains what novice gunslingers will likely be called to do to continue their training--that is, being part of various military organizations, possibly even acting as mercenaries. It makes sense that, because this is possible for talented people, a relatively down to earth character should be the one we look to. I would say that... John Wick and Big Boss are both characters that would fit this level of power.

Intermediate tactics are... conceptually possible. It is feasible that someone, somewhere in the world MIGHT be capable of this. For example, there are those out there than can cut BB pellets with swords after they've been fired; it's rare, sure, but it's possible. Using this reasoning, it might also be possible to shoot a bullet out of the air. It's VERY rare, but that has happened before, though only by accident. Given how rare it is, we can likely relegate this level of skill to action movie heroes, like Hawk Eye from the MCU or John Preston from Equilibrium.

Advanced tactics are... I'm not sure. A step beyond conceptually possible would likely be impossible to perform without superhuman characteristics. Curving bullets? Shots from unreasonable distances? Shooting a supernatural effect off of someone? Shooting through thick or multiple walls?

Expert tactics are... You'd be hard pressed to convince a person that this ISN'T magic. Not merely curving bullets, but having bullets do THIS to people.

Here's where I'm stuck. Who is better than the characters I mentioned? How much better are they?