i suggest u don't get too much into that. That's done in flash. It's actionscript has the gun as an movieclip "snap" to your mouse in a way different than the actual mouse... hard to explain.
It lacks the precision of your mouse. It's on fine grid, nevertheless, a grid. An example was when i got 3 bullets 1 on top of another right next to the center with a revolver. I mean literally ON TOP of, as in, it just looked like 1 bullet wound.
On top of this, anti-aliasing sometimes creates sort of a 'co-ordinate of best fit', which throws off your shot.
The only way to get around this is to put your desktop at a rediculously high resolution (to make the grid more fine) and set the quality of the flash to low(to get rid of the anti-aliasing factor).
Mind you, this all goes out the window when u use the "advanced" ones, since your not going for in-the-center accuracy anyway. But a new problem arises: In real life ur hand shakes randomly. In this game, it makes an infinite sign. entirely too predictable. In my opinion, that was just lazy of the author.
(this post is why u never link digital artists to other peoples digital work XD)