Use the approach which makes sense for how you're shooting at the time.
Approaches were a little difficult to wrap my head around at first. Unlike skills, they aren't about what I'm doing: they're about how I'm doing it. Any approach could be appropriate for shooting a gun, depending on the context of the action.
- Lining up a sniper shot? I'll Carefully take my time.
- Shooting a fancy chandelier to place the aspect Shattered glass on the floor? That's a Clever thing to do.
- Trick shots to impress a girl? I'm gonna be Flashy about that.
- Intimidating someone by shooting the wall next to him? I'm Forcefully taking control of the situation.
- Shooting the getaway car's tire before it's out of sight? I gotta do that Quickly.
- Shooting a blowdart from cover to quietly take down a guard? If I'm not Sneaky, what's the point?
When I narrate an action, I consider how I'm doing it, and assign the corresponding approach to the roll. Sometimes, though, it's not super clear-cut what the appropriate approach is. When that happens I have to analyse the situation and make a judgement call.
To re-visit the sniper example: Maybe I'm also trying to do it stealthily. But I can't roll Sneaky and Careful, so I have to decide which is more important: hitting the target or staying hidden?
Or the getaway car: If I'm trying to blow out his tire, do I have time to Carefully line up a shot, or must I do it Quickly and probably face a higher target difficulty to succeed without cost?
When I figure these things out, I then roll the most crucial approach. In the sniper example, the player probably gets to decide. But as a GM I'd likely rule that shooting out the getaway car's tire before they get around the corner must be done Quickly, since there's no time to be Careful--if I roll Careful on that action, I should automatically fail because I take too long (and that's just boring and frustrating, so it shouldn't be rolled at all).