Forgot to mention, you'll obviously need to find the exact point of the weapon so you cna make it look as real as possible, not sure how you'd do this.
Also, the sprite_dot is a variable that holds the id of the sprite "sprites/dot.spr".
You'll need to precache it and store it's id in a var.
Code:
public plugin_precache()
{
sprite_dot = precache_model("sprites/dot.spr");
}