It's a difficult thing to get right, whichever way you try, and takes lots of practice. The streaks from the gun smoke are very often depicted far too densely on models, in solid black. In fact, on the real thing, close up, although fairly dark, the staining is more of a dark grey, sometimes with a hint of blue (only just) depending on the ammunition, build-up of staining, and the colour of the surface.
Here's an easier way to experiment with, before trying it on a model. Get a 'Chinagraph' wax pencil or, betterstill, a ladies eye-brow/eye-liner pencil. Draw a very light line, about a quarter, maybe a half, of the overall desired length, then smudge this lightly in the direction of airflow, with the tip of a finger, extending the line as required. This can then be diffused further with the tip of a cotton bud (Q Tip), when using this dry, or wet, will give varying effects.
Try this first, a number of times, on a scrap of painted plastic, to get the feel for it.